Where do I even begin? I'm going to break down what I don't like about the game into sections to make this easier.
-I don't like the story and I like how the story is told even less. It is jarring to be constantly interrupted by so many in game cutscenes and converstions. It becomes aggravating to have to click through the same text on multiple playthroughs. I don't care about journal entries or audio diaries, especially not having already heard some of them six times so far. I just want to kill stuff and get loot. I'll leave the story alone because that is opinion and for spoilers sake.
-The UI for skills and the AH is disgraceful. I am a PC gamer and I demand to be able to see/select all my skills on one screen without having to scroll through them a whopping 4 at a time. Look to WoW for an example of a well done AH, specifically how the menu functions. Going through eight drop down menus is tiring and cumbersome. The item type should pop down like in WoW and we should be able to just check a box next to the affixes we want (more than 3) with a place to type in values, including min/max price.
-Sort AH items by buyout, have to scroll through 10 pages of auctions with no buyouts.....
-I am on A2 inferno and that may be as far as I progress because I am losing interest in the endgame very quickly. The items that drop for me in A1 inferno are mostly for toons in their 50s, some even low to mid 50s, and the ones that are 60 are almost all junk. I tried playing some of A2 and made it a little ways in, guess what I found? Crappy 50s items STILL dropping. Why?
-The legendary and set items in this game are generally terrible. In D2 I always had a goal of sorts for what uniques, sets, or runewords I wanted and it was "exciting" to find them. Now the uniques are weak and I just play hoping to find some no name rare or blue that has better stats of pretty much the exact same type that my current item has, which I did for the item prior to that one, and the item prior to that one, forevermore and unending.
-Lvl 60 class sets are pointless in their current iteration aside from novelty. They are overkill for hell difficulty and way underpowered for inferno.
-The blacksmith is a waste of money/time for all but the extremely lucky.
-There are so many junk stats/combinations it's rediculous. Everytime I find an item with str/dex or similar it makes me feel unhappy things. It is rather telling that one of the stats that gets me excited is pickup radius because it's rediculous to have to run exactly over everything without it, and there are few decent stats to want anyways. Attack speed, crit stuff, core stat, resists, vitality, movement, resource bonus, sockets, magic/gold find. Everything else might as well not exist as far as I am concerned. Most of those qualities can generally only be found on certain gear pieces, making the list even more narrow for those items.
-It seems as though there are seperate tiers of gear within lvl60. This is an absurdly terrible
idea if so. Weeks/months/whatever from now, who is gonna buy lower tier 60s items? Nobody because the AH will be saturated with 200 pages of A4 inferno rares/blues.
-With regards to weapons, mostly only dmg matters.
-I don't know if it is specifically your intention, or a side effect of some design decisions, but
I feel as though I rely extremely heavily on the AH for gear upgrades and have little opportunity to find/craft my own gear. Perhaps crafted items could be soulbound with a better chance to roll for useful affixes or some choice in what affixes are applied, while drops could either be much weaker or more powerful depending on the luck of the dice.
-A near complete lack of character development. Much like the story, the game holds your hand through that aspect and you are just along for the ride. It even tells you what skills/runes you unlock as you go, god forbid you be responsible for finding that out yourself. As if that isn't bad enough, we are forced to enable a "special mode" so that we can choose the six skills we actually want, instead of using skills from certain categories a la the default.
-One of the reasons for doing away with stat/skill system from D2 is that they wanted to make it so that all skills were viable. Noble intentions, but I would say that is still not the case. I play a DH and anything that is channeled, can't be aimed on the run, etc. is worthless later in the game. I use 2 attacks, Devouring Arrow and Nether Tentacles, and there are only a few that I would remotely consider switching to. So now we have no character development AND narrow build selection instead of just the latter.
-Some mobs are cheap and are able to instakill you in areas where you are otherwise progressing comfortably.
-I have died a few times because I was trying to click on the ground to kite and move around, but I was trying to run to a patch of ground that had a bunch of ranged mobs scattered about in a tunneled area. I am not even sure it would have been possible to click anywhere in that area of the screen. It is fun battling mobs, it is irritating battling the interface.
Diablo 2 was often coined as an action RPG and pretty much everyone agreed there was strong emphasis on the action, while it was somewhat shallow on the RPG aspects (not a bad thing). Diablo 3 further dilutes that RPG aspect of the game via non-existent character development, homogenized skills via % weapon damage, and few useful affixes for gear. While the comparison to Castle Crashers may seem like I am mocking D3, I actually enjoy both games. I just don't believe that is the kind of game D3 set out to be and it strongly reminds me of it. In addition, there are some interface issues, annoying storytelling mechanics, and upgrading gear without constantly turning to the AH is excessively difficult.
Prior to D3 coming out, I had planned on it being "my game", in that I was pretty much going to quit playing everything else and play only it for quite awhile. In its current iteration, that is definitely not going to be the case. I will still play it casually leveling toons with friends and once in awhile by myself, but I have already pretty much had my fill.