5. Roll-playing Game
While not as bad as some systems (*cough* World of Darkness) in terms of the amount of dice you roll at one time, there are a lot of rolls for resolving single tasks. There are so many extra rules for things (most of which require rolls) that the emphasis is on the rules and dice rather than the roleplaying. You also need multiples of every type of dice - while most roleplayers have these it's still a bit of a pain. One roll per action - surely that's enough for any system? Oh, and don't forget your synergy bonus!
While I realise that alignment is just supposed to be a guideline, the rulebooks seem to contradict that. You can't play a Paladin unless you're Lawful Good at all times. How do you make a Paladin interesting then, while still remaining a Paladin? They're all going to be shining paragons of justice and virtue. Playing clerics requires adherence to certain alignments, and the same for some other classes. While it might not be constricting for some, to me it's like someone pigeonholing me simply because of my ethnic background or my accent.
Now that I'm a 3rd Level Rogue, I'm more likely to survive being stabbed that I was last week, when I was a 2nd Level Rogue. Having arbitrary levels which state what abilities you have or can have is so ... 1980s. Please, I thought we left that all behind when we left high school. OK, you can use the optional rules (more rules!) around training, to make this a bit more realistic, but still. It bugs me that I can't be (for example) a Wizard who only knows a Magic Missile spell but can cast 5 of them instead of the 3 dictated by his level. It feels like exactly what it is - a completely arbitrary way of rating characters so you can pit them against random monster enounters (hmmm .. I should have put random encounters on the list too).
2. Two Book Minimum
You want to play Dungeons and Dragons? Then you have to buy at least two books - the Players Handbook and the Dungeon Master's Guide. Oh, you wanted a background? One more book (e.g. Player's Guide to Faerun). Oh, you needed monsters for the background. That's another (e.g. Monster Manual). Brand new at MSRP that's $122.80. Even getting them second hand it's over $50. One list of "essential" D&D books on Amazon has 40 entries totalling $834.75 (that includes Amazon's discounts).
"But Wizards spawned the whole d20 movement, when they 'open sourced' the system!" Oh, really? Great - lots more games based around the same shit system.
1. Rabid Players
So, after spending vast quantities of money on all these expensive and extensive rule books, it seems that the average D&D player doesn't want to be told that the game is pile of poo. Instead, they defend any slight vociferously, even when blatantly in the wrong (or when they miss the point entirely). And this symptom seems to spread into the other popular D20 games (ever notice how often Mutants and Masterminds players say "System X is no good, you can do all that and more in M&M"?).