I know some of the big issues such as itemization and class balancing are still very much a work in progress, but lets take a look at some other things that I feel deserve some attention.
Face-tanking vs Ranged
Today I ran into a pack of no fewer than 30 Cloaked Writhing deceivers playing mp8 on my Demon Hunter. Oh boy here comes trouble! In D3 it doesn't matter what class you are. Sooner or later you end up face-tanking. Even if you don't face-tank, Face tanking is just far more efficient no matter what class you play.
I find this to be a huge problem as it kills the diversity in play-style. Add to that the hit check mechanics where you take damage once a monsters attack animation has started whether you put 2 screens in-between the offending monster or not and it's just far more efficient and safe to face-tank. Its really time for this mechanic to go belly up especially with the increased mob density. No blizzard - It's not fun getting killed by a monster 2 screens away because I got caught in its attack animation. It almost seemed as if you are discouraging player from playing ranged.
I honestly feel that at this stage in the game damage should be equalized for all classes. You want to be a tank you gear for it no freebies. You want to be a glass cannon, same goes. In original inferno monsters did insane damage and did not have nearly has much HP so it's about time damage is equalized. The hit check mechanics for physically avoiding hits need to go as well. It is a bit far fetched to suggest that ranged need to avoid attack animations (not hits) attack animations from a screen full of scores and scores of mobs being that they are less durable by design. All these factors go into making d3 all about face-taking for the most part.
Monster Power & HP
I think monster power in some ways was a mistake - It unbalanced the game in many ways. The inflated HP of mobs just promotes face-tanking for one thing. I often kite with my DH's and wizard using builds that rely more on burst damage. With the now improved monster density, kiting could be very frustrating / tiring as there just isn't enough room and skills which seemed to have been balanced around the original inferno lvls of HP, just don't do enough damage to keep range. Don't get me wrong, I love the increased mob density, but as I said, it really seems like skill damage is based on earlier inferno monster HP lvls. To add to this dilemma, the gap between Cookie cutter and non-cookie cutter builds could be quite extreme in the latter MP's.
Lets take for example CM wiz vs non CM wiz or on the extreme end a CM/ww/sns wizard. To begin with, there is a massive gap between playing a wizard with CM vs no CM. All of the wizards defensive skills have a cool-down. Only certain skills have to proc rates to really take advantage of CM to lower those cool-downs. I know devs say they want to keep play-styles different. But when your options are basically to suck or to not suck. There is a real problem there I think. The gap really needs to be brought down between cookie cutter and non cookie cutter builds. Proc rates need to be improved. When you have something as important as Defensive cool downs and Critical mass relying on proc rates. It's only natural that everyone runs to the skills with the best proc rates.
But back to the topic, I'm not really sure what can be done about this. When I play all classes at mp4, where the HP is close to what it originally was to inferno. The seem a lot more balanced. But when the HP gets ramped up there starts to be a greater divide and less and less build diversity. The problems brought out by Monster Power are only exasperated by my next point which is:
Lack of Customization -oh no not this dead horse again.
Yes this dead horse again! This is one area where I think d3 suffers a lot. The lack of a skill tree / any character customization outside of gear makes for a very shallow game. Yes its convenient and easy to have every thing in that dept to just be handed to you on a sliver platter, but it takes away a lot from the depth of the game. Without skill progression outside of how much damage your weapon does / limited gear affixes, skills have to be balanced around the 44 MP difficulties that now make up d3. (Normal to inferno - mp0-10) This lack of fine tuning, no matter how you try to deny it makes the d3 experience a very shallow one. I'd even say there are no builds in d3. Only load-outs.
Also, without skill progression to limit skill damage a skill that does xxx% weapon damage may be overpowered right up normal right up to mp6 or so inferno where it suddenly becomes utterly under-powered. This is an example of a problem that culminated with MP adding 44 difficulties to the game and increasing HP, no skill progression with a lack of customization.
Many utility skills see zero progression all together as they don't gain anything from affixes. Add to that that many skill runes barely have any noticeable difference between them.
An example off the top of my head. Magic weapon - Conduit rune. Attacks have a chance to restore 1 arcane power.  arcane power? How could this possible do anything to change my play-style...or how is add anything to my game-play? This skill never gets any better from the time its unlocked. Add to that the low proc rates for most skills and there really is no reason to use this rune.
If you're not going to have skill progression the traditional or in anyway, then skill runes need to be really diverse. Saving 2 resource cost on a skill, or even worse, having a chance to save 2 resource cost is not fun nor is it diverse.
Players like to customize their characters and feel unique Devs. Right now it's all about the gear and cookie cutter gear and skill setups reign supreme. It's really hard to get that gear through farming. Only other option is through RMAH and gold, which is decreasing in value thanks to dupes / what ever exploits and scams are out there, There is little option to fill the gaps in your gear by customizing your character via Skill and stat progression. I know in d2 a lot of people put enough stats to wear their gear and put the rest in vitality. But I was one of those that never went the cookie cutter way. I liked making hybrids and getting my character to play just the way I wanted it to. I find a lot of that is missing in d3 thanks to the lack of character customization - It again makes for a shallow experience.
I'm not suggesting bringing back the old skill system. As you guys will defend to the death that it is dated. But I feel the lack of character based progression is a problem in d3. It would be nice to have a way to progress our skills outside of gear and super expensive weapons.
Monster AI and Combat
I think there are many good things about to combat in d3. It's smooth you need to combine skills and the art is well done. But there is much that can be done. I think that this is a HUGE area that could be improved on to add more depth to d3.
Right now monsters are just giant HP punching bags. And for the most part they are fairly stupid. Most of them try to run up to you and beat the crap out of you despite the fact that you may be a ranged class or tank. Here are some things I feel will go a long way to improving the combat depth in d3.
Monsters should de-buff players more.
One mob that I really like are terror demons which are sadly only in Act 4. They can get the jump on you and use terrorize on you - cutting your healing by 75%. This is one of the most dreaded monsters in the game I think. It's fatal if one jumps you while you're fighting off a large mob and it forces you to change your play-style on the fly. A lot of the de-buffs other monsters have are barely noticeable by comparison and don't make you do anything to play differently. I think you should keep this in mind for the future.
Life-steal should not be such an end-all solution to healing
I think it would make things a lot more interesting if life-steal had effectiveness reduced against certain monster types. Like undead. I mean these things have no life in them. It would also promote having a more balanced character with multiple sources of sustain.
Monsters should have Resistances
Right now elemental effects serve zero purpose in the game except for cold damage. It doesn't matter what elemental weapon or elemental skill you choose. If monsters had resistances it would certainly add depth to the game and certainly to group play where having a varying selection of elements would be to the advantage of the group and promote team play and you won't be-able to steamroll over every single mob type in the game with the only difference being how much HP they have.
To go along with this idea. Elemental effects should do something...all off the top of my head.
For example - Fire, AOE and non stackable DOT. Most effective against normal monsters.
Poison - Longer more potent stack-able DOT - Most effective against defense type monsters
Cold - Freeze and slow to varying degrees based on weapon lvl.
Holy - Bonus damage to undead / reduces damage from undead.
Arcane - Slows attack / Reduces damage. Most effective against Normal monsters.
Lightning - Chance to chain your attack to multiple targets and to paralyze monsters.
Monsters could react differently to different classes
It doesn't matter what class you are, monster attack you in the same way. Run up to you try to beat you to oblivion. It doesn't really add any depth to the combat.
Speaking from my main class for example, the Demon Hunter. One thing that has bugged me is how its impossible to be sneaky for example on a class like the DH which has the whole theme of shadows. There is no such thing as stealth. Monster sees you...your positioning is relatively irrelevant. Terrain and shadows / time of day on the map do nothing to affect monster AI in that matter. Attacks aren't really stealthy - you kill one monster and the whole pack bears down on your exact position without getting confused / surprised or anything.
There should be Monsters that are immune to certain CC effects
All monster for the most part can be CC'd the same way. Again it makes for a lack of variety and depth. Just a thought to mix things up outside of simply buffing / nerfing skill damage.
One of my biggest gripes with playing is a party is when there is a wizard freeze locking everything. Yes it makes things easy. But what does that do for the heavily tanky builds or the aggressive ranged kiters in the party? Every fight ends up playing out the same and their is no need to play at what your character is good at. It would make things interesting if there were more monsters that were naturally immune to being slowed, frozen, stunned, knocked back and that it was just consistent across the board. Again this would be a huge benefit to the quality of group play in my opinion.
IN game Effects in party play
With the Improved mob density, this game could be come a real jam on screen. I've recently found myself dying simply because I just can't see anything! I think skill effects need a bit of transparency and /or less saturation. 4 party games are total chaos. Large monsters and exploding skill effects often completely block out your player. Not as pressing of an issue but I feel its important none the less.
Well this is my blurb on how d3 could be made into a game with alot more depth. Feel free to add your thoughts but lets try to keep it civil.