Alright, after playing Defiance for bit, my two cents...
This game is heavily flawed. The AI isn't great, as far as shooters go, the appearance customization is terrible, and I've encountered a fair few bugs so far, some preventing me from turning in some of the rather tough-to-solo Blood of Heroes side events. The graphics are functional, and as far as MMOs go, they're great, but still nothing to write home about. Also, the lack of complex location based damage puts it behind other shooters. Not to mention that the absence of classes and limited race options mean that Alts aren't much of an option.
That's the bad news. The good news... this game is %$^&ing FUN! While the shooting mechanics aren't amazing, they're oddly fun to play with, and I would say they're about on par with Borderlands' FPS mechanics, which weren't stellar either. But the world is very interesting, and it's cool to play in a real world location (albeit terraformed). That was one of the things that pulled me into The Secret World. An MMO with a boring world is no fun to explore, and this looks like it'll be real cool. The combat, despite the mechanical flaws, isn't the chore it is in most MMOs. In WoW, TSW, EQ, and GW, I would charge from location to location to avoid combat. It was boring. But in this I find myself looking for a fight. the sidequests are run of the mill, but still fun, as are the random events. But that's not what sold me.
Arkfall. It's amazing. I've only played for a day, and I've already taken part in a raid-sized event, and gotten a ton of loot. If The Secret World mastered the Instance Dungeon, Defiance gets the prize for World-Raids. Check out a youtube video if you're doubtful.
So yeah, it's a bunch of individually kinda crap mechanics and tropes cobbled into what ultimately comes out to a flat out awesome game. It shouldn't work, but it does, wonderfully. I'm gonna be playing this one for a while. Thanks to the OP for the recommendation.