This game is built on the City of Heroes engine, and as such acts more like an expansion pack moreso than it's own game. That said, the game is still fantastic. Each Archetype (class) is unique from their Hero counterparts, and actually a whole lot more fun to play. The Villain Archetypes were all built around using their Inherent power, which is really the only design flaw in City of Villains.
Building around the Inherents means that Brutes, for instance, have a lower base damage for their attacks, because the more they fight the more Fury they build, which adds to their damage. in the PvE game that's how it works, but in PvP Fury is all but useless.
That said, City of Villains brought zone PvP to the "City of" franchise, rather than having global PvP the game (and Heroes) has 4 zones which are strictly dedicated to PvP. Each zone has a level cap, and a minimum level to enter, and when you enter that zone you are set to the maximum level for that zone.
The game's mechanics (along with Heroes) are something unique here that I haven't seen in many games. The game effectively employs 2 different levels for your character, one "Security level", and one "Combat level". Your Security level can only go up, but it will only raise when you visit an NPC trainer upon leveling up. Your combat level can fluxuate, if you Sidekick/Lackey to a higher level your combat level becomes 1 less than theirs, while Exemplar/Malefactor will bring your combat level down to match your partner and lock any powers above that level that you have.
The biggest thing that I would say I don't like about either game is that in order to get to know the story behind things you really have to look for it. The game has an extensive backstory, but it's not presented to you unless you seek it out.