1. The graphics are... well... weak. Not the atmosphere or special effects, but the polycount and models. This game doesn't look much better than Titan's Quest, which came out several years ago, and was created by an indie developer. Graphics don't really mean much to me, especially for a rogue-like (I play ToME regularly), but I'm just surprised this was the best a AAA game could muster. I hope Blizz comes out with a high definition patch like Skyrim did after launch.
2. There's no zoom on mouse wheel. Sure there's a zoom button, but that zooms way, way too close to be useful. Sometimes I want to get a better look at the action, but that's not practical when my choices are either bird eye or right in my face zoom levels.
3. No quick weapon switch button. Really? The other two issues are just for vanity, but this affects game play. I can't switch to 1-handed and shield in tight moments quickly, and that can cost you a life in hardcore.
4. Bad AI. I've had melee monsters just stand in a spot and get pelted by arrows. Those big charging monsters in Act 1 seem to do this the most. It's not when they're preparing to charge either, sometimes they just don't even bother to look in my direction or do anything at all when I see them on screen.
5. Way too much glowing particle effects. Turning on low-FX didn't even change a thing visually. Maybe it's fine for a wizard to have every left or right click send sparkles and neon bolts across the screen, but it's just weird as a Demon Hunter. Most attacks don't even have an arrow or bolt graphic! I know that this a subjective complaint, but it's still something that bothers me.
6. Horrible latency in single player. I hope this is related to the server shenanigans and will thus be fixed, but I've been getting 300-500 latency in single player! It's somewhat ridiculous when enemies teleport across the screen before your attack can be registered. I'm using cable internet in a metro area, so this shouldn't be a problem.
Like I said before, I'm enjoying Diablo 3. I recognize it has shortcomings though, and I think it's important that the community recognize these as well. Hopefully, Blizz will be able to address these problems in patches... once they get the servers fixed.