Crosmando: It's because MMO's, the Dota-like games, and competitive online multiplayer gaming in general, are the
ultimate form of meta-gaming.
By meta-gaming I mean, there's no sense of immersion, the fact that MMO's are in a fantasy setting means almost nothing to the average MMO player, doing a quest doesn't mean going on an adventure, it means 456 xp, casting a certain spell doesn't mean some fantastic event, it means 47 damage. It's like a sport, only online, that's why the "communities" are so awful, who on earth would even want to breath the same virtual air as morons like that is beyond me.
They should also drop the "RPG" part from MMORPG, there's no role-playing at all in these games. If a game is doing something right, it should feel less conscious to the player that he/she is in fact playing a game, doubly so for role-playing games.
Magnitus: I am of a similar mindset as you on this (to an extent, I will exploit overpower mechanics that I see as long as they don't force you to play in a manner that I consider absurd and if the overpowered mechanics make the game too linear, I will stop playing).
Watching several of my friends playing MMORPGs, it felt like they were playing less of an RPG and more of an open sandbox (with all the sharp corners smoothed... not too many changes: very status quo) with a story on the side delivered to each player in parallel.
And yes, many players will just actively look for exploits (even the completely absurd ones) and limit the possibilities of the game to revolve around those exploits.
However, I attribute this more to a difference in taste than a difference in temperament.
While for many, this "natural selection" mindset will also taint their social dealings with others (ie, "pwned you nub!"), others will compartmentalize their views better and treat you nicely even thought they approach gameplay in a manner that you consider off putting (some people are just very efficiency minded and can't turn the switch off, even when playing in the most efficient manner possible completely destroys the flavor of the game, but that doesn't make them brats or morons).
Imbalance in RPG's has always been a killer though, recently in an interview Chris Avellone (whom I'm sure you're aware of) said that the most important thing in developing an RPG was balance, because you only need to overpower
one spell or skill and that makes using anything else pointless.
But I still think games can be
overbalanced, that is to say that everything is so minutely balanced that's impossible to create and roleplay a character which isn't just a carbon copy of its counterpart in another class/race (isn't one of the reasons
Dungeons & Dragons 4.0 disliked was that it's too balanced?). Their needs to be balance, but room for flexibility so you or your enemy can pull surprises out of the hat.
But back on topic, I did actually play WoW years ago before TBC expansion came out, and I did have some fun experiences with other players, as I understand these days you can auto-que anywhere in the world for a dungeon, and it teleports you there by finding other players from different servers (what bigger immersion killer is that right?), but when I played you could only go into a dungeon by physically going into the entrance of the dungeon with your party. Some fun times I had when we got lost and completely alone in a dungeon.
But apart from dungeons I spent the entire game just adventuring around on my own, never really spoke to any other players.