The following is a list of what I feel are Diablo 3's major shortcomings and why I am personally very disappointed with the game. The following are just my own personal opinions, but are clearly shared by many others. I finally took the time to think about them, point by point and write out what I didn't like. They are not ordered in any particular way, other than just how they popped into my mind.
1. The items are boring and uninteresting
Diablo games have always primarily been about the item hunt, there need to be super interesting items to chase that give you upgrades, fantastic new abilities, maybe even ones that will support a specfic powerful or whacky build. As long as the item system is subpar, the game will be subpar. Diablo 3's item system is about as boring and generic as you can get. DPS, Main Stat, Vit, All Resits, Crit GOOGOGO. Every class, every item slot. There are never interesting decisions or choices to be made. You are simply increasing the numbers. Nobody gets excited about upgrading their boots +1 dex but that's the type of upgrades you are forced to seek out in this game because there is nothing else to chase.
2. No character investment
Auto-stat point allocation, all skills and runes available at all times, no permanent decisions, your character is just boring and you don't feel like you had any hand in making it, in fact you didn't make it, it made itself. You have no connection to or investment in your character at all. You are walking gear. In Diablo 2 you also had incentive to make characters over and over again, You had a frenzy barb, a WW lance barb, a WW sword bab, they all played differently and needed different gear, same for your ice sorc, your fire sorc, etc. This incentive doesn't exist in Diablo 3 which effectively renders the game 1-59 completely useless, including all low level legendaries and such.
3. No social system
The lack of a decent lobby and the ability to create named game instance to attract people with similiar and desired interests (duels, trades, questing, etc.). Just trying to give away items is painful, spam chat, hope to find someone who wants it, invite them to a party, create game, etc. Having a list of named games that people could pop in and out of to do whatever was in the title was awesome and is sorely missed. It allowed people to group with those who shared similar interests, rather than just 3 randoms. The 4 player cap is also horrible. 8 players in a game would be crazy and also FUN. If we can't have 8 at least give us 5 so we can have one of each class. Why don't you let people CHOSE how they want to play? Why was 8 players deemed "not the way it was supposed to be played" and removed? Let people play how they want, be it solo or in a huge group.
4. All game aspects have been simplified/streamlined/removed
Remember when weapons had range? Like using a lance was slow but had long range and using a mace was faster but had short range, remember that? Remember when axes did extra damage to demons and maces did extra damage to undead? Remember when each of the elemental damages actually did damage differently? Remember when there were 6 different gems types and they did drastically different things depending on where they were socketed, making all 6 types interesting and viable. Remember when there were more than 4-5 types of shrines? WTF happened to all that? They were all small, simple things that gave the game character and at least a little depth, why were they all ripped out? Worried it might confuse people? Diablo 3 has been streamlined, simplified, and flat out dumb-downed to near death.
5. The story is terrible and is shoved down your throat.
Diablo games have never primarily been about the story, the story has always been kind of a back drop and left a lot to your imagination. However Diablo 3's story is not only ridiculous and childish and laughably stupid, but is CONSTANTLY IN YOUR FACE ALL THE TIME. You can't get away from it, you are always questing, no matter what, replaying the same crap you've done a million times over and over again. In D2 once you got done with the story you were done with it if you chose to be. You could kill all the bosses or uniques, go where you want, etc.
6. The world is not random or open enough
The world feels small and disjointed, the areas are all connected the same way geographically every time. There are no large open areas that are random and connected to random other areas. The ability to warp between acts is gone. The entire world feels railroaded. This is very related to point 5. It also makes the game feel much shorter and smaller than D2 when it actually isn't. For the love of God give us the ability to play the game in "open" mode, allowing us to use all waypoints and go anywhere without questing. Make the world areas significantly larger and more random. Give us the ability to explore again.
7. The AH makes the item hunt feel pointless and boring
No matter how much you farm, better items are always a few clicks away on the AH, it's always more efficient to use the AH. Finding awesome items as drops will always be way more fun than buying them Wal Mart style from the AH, however farming is just a gigantic inefficient waste of time. Knowing this kills the fun. The AH clearly isn't going away, but something needs to be done to promoting FINDING items, not just buying them in the AH. Unfortunately I have no answer on this. Find a solution though if you want D3 to be fufilling despite the existence of the AH.
8. The world is not as dark/gothic/mysterious
Yes the old art controvesy is back, in fact it never went away. I know this has been said a million times, but the game is NOT as dark as previous Diablo games, I'm sorry it just isn't. It's too cartoony and it really detracts from the game. A lot of this is due to the horrible story (see point 5). There's way too much dialog breaking the mood, way too much comedic relief, villains that behave like petulant 4 year olds taunting you, not to mention the pastel haze over everything. Azmodan and Diablo aren't scary or mysterious, they're inept cartoon villains. My character or followers constantly spotting one liners doesn't make me smile or draw me in, it KILLS THE MOOD.
9. The music is basically non-existant
The actual music tracks that are present are ok, but for some reason it was decided that 95% of the game it should be bland background ambient noise rather than discernable tracks and it makes the game feel dull and lifeless. I know you don't have access to the talents of Matt Uelman anymore, but surely something could have been done more interesting than barely noticable background rifts. Music creates mood, followers spouting off stupid lines do not.
10. The level cap
It took many months upon months to reach the level cap in prior Diablo games, you always felt motivated to play because you were always leveling up. So if you were farming items and not getting anything good, at least you were leveling, your character would improve, you'd get those 5 stat points and shiny new skill point to spend, grinding to get it gave you a sense of progress, no matter what was dropping. The paragon system has tried to address this shortcoming but is still not as rewarding as regular leveling would be.
11. The drop rates are terribly out of whack breaking the slot machine effect
In D2 you were getting uniques and sets frequently, maybe they mostly sucked but you were still getting them, and were possibly useful for your low level characters, which you would DEFINITELY be making over and over again.. It kept the carrot/slot machine effect in a positive way. In D3 legendaries and sets barely drop at all and RARES DROP TOO MUCH because they are normally crap. Items should drop with frequency proportional to their power, but can't be an extreme where they drop too frequently or too infrequently as it breaks the slot machine effect. Finding the "sweet spot" for drops is a delicate task, drop too much, it's boring, don't drop enough, also boring and frustrating. I'm not sure what the sweet spot is, but D2 felt right, D3 does not.