One of the things I've had to get used to playing Bryony is playing an "aligned" character for the first time in Achaea: she's Good, with a capital G. And this got me thinking to the different alignment systems I've seen.
Most RPGs have alignment systems based on D&D in some fashion or other, I think. In D&D, you're either good or evil, or neutral between those two, and either lawful or chaotic, or neutral between those two. These alignments determine what sorts of actions you would consider acceptable or not, which is especially important for certain classes. If a paladin does something evil, he loses his paladin skills until he has formally atoned for it.
I don't see this sort of thing often in graphical MMORPGs, but many MUDs have something similar hard-coded into the game. The way this often works is, every mob has an alignment. By killing good things, you become more evil, and if someone casts "detect alignment" on you, they'll see an evil aura. Similarly, killing evil things makes you more good. If you kill a mix of things, you'll end up neutral.
So, I played one MUD once where what you wanted was to stay neutral, by killing an equal mix of good and evil things. The further you got from neutral, the more often your attacks would miss. That was pretty weird.
But usually, the way it works is that alignment doesn't matter for most people, but members of certain classes might need to maintain the right alignment (priests have to be good, necromancers have to be evil) to use all their class skills.
Achaea used to have a hard-coded alignment system like that, but they deleted it a couple of years ago, which I think was a good choice. It's just not very realistic. Killing a bunch of bad people doesn't necessarily make you a saint, for one thing. For another, being able to determine moral issues by just looking to see what kind of aura a person has is an overly simplistic mechanic that can end up interfering with more serious roleplay.
The Baelgrim Fortress in Mhaldor, the city of Evil. This artwork is by player Shindelik.
So Achaea now has, in my opinion, the best of both worlds. On the one hand, there's no weirdly artificial hard-coded alignment system anymore. But the game kept the good part, which is that your actions still have roleplay consequences. So if you're a priest and you're caught slaughtering innocent villagers, you'll get kicked out of your city, and the Citadel of Light will excommunicate you so you lose certain of your class skills, until you change class. And if you're a necromancer and you're caught working against the gods of Evil, you'll be cut off from your necromantic powers. But none of that is hard-coded: the gods appoint an Archprelate and a Dread Ecclesiarch to make these decisions, so rather than being a pesky mechanic, they're a source of roleplay drama and depth.
I think this minor restrictions are a good feature of the game, because it makes the game feel more real and exciting when there are long-term consequences for certain things you choose to do. And it means that the different cities have different classes available to them, which makes city-versus-city combat more strategically interesting. Check out Achaea, a no-download MMORPG, and see if you agree!