Though I am not yet in Inferno and haven't faced the brick wall others are talking about, here are my 3 main concerns Diablo 3 needs to start to improve upon for me to continue playing:
1. Maps need upgrades.
I'm just about to begin Hell mode on my DH (Demon Hunter) and I already have a general idea of where things will spawn and the general layout of every map in the game, prior to even starting to unfog and explore it. For instance, I really wanted The Tide of Battle achievement in Act 3 Battlefields so I would log out/in, zone back to bridge, hit points A, B, and C in a matter of minutes to verify locations, and repeat if the event was not there. It seems to me there is a core map for every zone like a basic template and the randomness boils down to what events/entrances appear in set locations rather than the map varying in size or overall layout. Also, I can zip through a map on my alt characters ridiculously fast, finding exits and required locations without really having to search. For example, when doing the Search for the Crown quest on my monk I knew where Defiled Crypts would be, popped in to see which crypt it was, and if it did not resemble the location of the crown, I left and tried another. On three of my new classes I'm leveling I found the crown in either 1 or 2 peeks into the Defiled Crypts before clearing them and receiving said crown. Any improvement in randomness and map size, as well as inclusion of more events or places to search, is direly needed as this is a large concern of mine in terms of whether or not I will continue to devote time to playing this game.
I play World of Warcraft, am a long-time fan of rpgs/action rpgs, and adore dungeon crawlers, so farming for loot is not a concern of mine. In fact, smashing maps for potential upgrades is part of what draws me to a game. Having said that, there's no real incentive to farming in Diablo 3. Most of my drops, including the couple of "legendaries" I have found have been piss-poor at best. If I repeat maps, it's simply to gather gold to buy my upgrades from the auction house. Likewise, most of my drops (with my follower decked with Magic Find) are for levels well below me, 10 or so levels, and even if the stats are decent, this gear either sits in my stash for an alt or goes on the auction house for some quick gold. While I haven't experienced it for myself, I have read repeatedly that you can not farm gear in Acts 1 and 2 of Inferno in preparation for Acts 3 and 4, making the grind for gold, not loot. This is a terrible flaw in a dungeon crawler, centering the game around buying/selling rather than exploration/drops. To advance, you must play the auction house, which requires leaving the game entirely and sitting on the character screen, refreshing, and searching for decent prices on required stats. Along these lines, the Blacksmith is a pointless endeavor. It requires a substantial amount of gold to upgrade, as well as mats, yet offers weak, randomized gear in return. Again, the auction house will always have better gear than what can be crafted. It's somewhat useful for alts in beginning stages, but beyond Act 1, you're better off spending 10k gold in the auction house instead of 10k gold unlocking new recipes.
3. RMAH (Real Money Auction House)
I still have my fingers crossed this feature does not make it to live. This is less of an improvement and more of a suggestion to keep it out of the game, for the RMAH will most likely kill the gold auction house, which I said above is necessary for advancement, and require a player to sink real-world money into pixelated gear. Not to mention it will open the floodgates for hackers and gold spammers (that are overwhelmingly present already). Personally, I will not spend a dime on such a feature, though I'm aware others will. I fear the day this is implemented, because there's a great chance that it will become the sole way to advance through the game, meaning, in a sense, Diablo 3 will be a pay to play game. I suspect already that people in Acts 3 and 4 Inferno, people farming the hell out of them (pun intended), have a stockpile of powerful items they plan to stick on the RMAH for profit, instead of gold, especially those people that discovered chest exploits to bolster their gold in the initial days after launch. It will also offer a monetary medium for hackers to sell stolen, overpriced goods that players sunk hours to obtain.