Alignment. I think the word delights some roleplayers and sends others running for the door. I think that one stat may single-handedly be responsible for causing some roleplayers to decide that D&D is overly simplistic or immature in nature. While it’s true that it’s an overly simplistic view of morality, as a mechanic for certain in-game effects, simple is better. Unfortunately, it’s really the manner in which it’s been portrayed and used that has caused so much trouble, rather than the mechanic itself.
One of the staples of the fantasy milieu is the concept of good versus evil. This manifests as abilities whereby, say, a holy paladin harms the minions of evil by his very presence, or can feel the evil emanating from them. The alignment system provides a means to accomplish this sort of thing, and it can also be used as a way to get a rough handle on how your character views authority, good, evil, and so on–which are useful things to know in a fantasy campaign. In my opinion, these are the reasonable uses of alignment.
Then there are the folks that use alignment to constrain personality in artificial and restrictive ways. For example, there’s the DM who feels that characters can never perform a single little act that “violates” their alignment. Sure, paladins, for example, need to remain “pure” in order to be holy and thus keep their powers; that much makes sense. But most people’s actions do not adhere rigidly to a narrow line of behavior–instead for most people alignment would be an average of their actions, a corridor that they mostly stay within but occasionally veer from. Even that paladin might occasionally do something a bit selfish or cowardly and then need to atone for his actions, for example.
Then there’s the example of the player (I’m sure everyone’s had one of these at some point) who plays a chaotic neutral thief, and then steals from the party because he has to, because it’s somehow dictated by his alignment. Nonsense. There’s nothing saying that a thief who’s generally greedy and selfish can’t be loyal to his party out of either personal friendship and loyalty or simple intelligence (gee, stealing from your friends might just be counter-productive). Once I actually saw a person do a good job at playing the thief who stole from the party; he only stole occasional items, only stole things that would work well for him, and had interesting personal struggles at times over whether to subsequently bring an item into play because he cared more about a party member’s well-being than he did about his own wealth.
People also tend to have very narrow definitions of what good, evil, law, and chaos mean, and allow those to color their views of alignment. For example, the minions of an evil overlord might be lawful neutral–not evil at all–because they obey him out of duty, not a desire to do evil deeds. An assassin trained to kill evildoers who have somehow escaped justice might be lawful neutral, because she sees herself solely as an instrument of the law.
This is up for debate, of course; to some people, assassination should be ranked as evil no matter what the motivation, as should the carrying out of the evil overlord’s demands. However, I definitely prefer a more perspective-oriented view of alignment. It keeps your players from always being able to pick the overlord’s minions out of the crowd with a simple “detect evil” spell, and allows characters to be more interesting.