1. Gill pretty much nailed it there, I've yet to feel like I'm grinding, and from beta experience that carries into the higher level tiers.
2. No mindless auto attack button mashing here. There is an autolock on feature, but with the pure amout of enemies, battlefield awareness and possible adds you get, it is probably the least used aspect of the game. Unless you use a shotgun, you actually have to target ala Battlefield to hit something. Enemies will move and dodge....cover matters as well as being a moving target. Kneeling gives you a damage bonus, but also makes you more vunerable. (unless you're behind cover..

)
- So in a way, combat does rely on normal attacks, but each weapon uses a different damage type which different enemies are resistant to. So you may use the LMB and shoot the same, but it does take thought in what you're using and when you're using it.
3. Like Gill said, spies are glass cannons. Just SICK damage with the sword and while their abilities don't translate well to PvP at the moment, for PvE you can just rock.
Polymorph can let you take on the look and ability of your enemies. Mag flash can blind them reducing their damage and accuracy. Traitor.....skip that one.
Sniper also is a high damage/lower health type character that takes technique. You can one shot kill almost anything in the game from a distance, but you have to plan how you're going to handle the aggro from the things pissed off buddies.
Honestly, L1on I think you'll love the game. I haven't had this much fun since the early days of
WoW and I'd actually say I like TR better.