There are quite a few articles and rants on the web explaining why Dungeons & Dragons sucks. So, let's take that statement at a face value — D&D gargles balls, plain and simple.
That said, most of the arguments I've seen focus too much on the details. They talk about alignment, about how classes are set up, about the amount of rules to handle or the number of books you need to buy. Some even point out the player mentality which, granted, is greatly damaged by the play style the system encourages, but I've never really seen anyone cutting the issue to the bone and getting to the core. Here's my attempt to lay out the four main reasons I don't use D&D:
>1. Power level.
It would make a lot of sense for a system define a certain power level and stick to it throughout the game. In D&D it’s impossible, because if you want to improve your character in any way, you get a damn package of abilities with every level. The guard dogs you feared in the beginning of the game will be helpless pups after a couple of “adventures”.