I had to spend some time thinking about this question. After all, I spend a fair amount of time running/playing D&D, so I really don't
hate any part of it. But then it hit me, so here's my thought:
I abolutely hate, with the intensity of 10,000 suns, the balance of the game when it comes to running out of resources. Here's what I mean: you're moving through the dungeon, fighting things and taking their stuff and so on, and then you have to artificially stop and rest for 8 hours every so often because the wizard/cleric/bard and so on are out of spells and have to reload.
Now I know why it's done that way: characters are balanced by some being able to have more power, but only for a limited time, but it completely blows my suspension of disbelief. I can't think of a single movie or book where characters did the "stop, rest up, regain power, get back in there" scenario.
It also makes some kinds of scenarios very improbable. For example, I played in a game recently where we had to rescue a kidnap victim. We snuck around many of the bad guys, but got into several combats that were seemingly unavoidable. By the time we were about to get to the main combat we were spent! The group argued that we should find a place to hole up and rest, so that our spellcasters could recover their spells! We were on a time critical mission and the game broke down into a discussion about resources!
So that's my beef. Aah, that feels better to have said!
--Steve