There's a few ways to invite someone to a group but the easiest to explain on here is type /invite (or just /i) and then the player's name. ie:
/invite anawin
then he clicks accept, and you guys meet up nearby and head into the cave to kill droids, or whatever.
any quest marked heroic 2 is best done with another player. some are marked heroic 4, which means you'll need 3 or 4 people total
Don't lead with force leap (or if you do, force push very shortly afterwards). He does a knockback followed by that really nasty blast, which is on exactly a 15 sec cd. So every knockback run at him mashing force leap as hard as you can
Since the cooldowns match exactly, you can interrupt every single one like that. Otherwise just kick his other stuff when you can, and wait til after a knockback and after using kick to use your master strike. Without his knockback blasts he doesn't hit especially hard.
survivability without soresu is so low that I'm fairly certain most dps guardians don't use their dps stances anyway, heh.
in the long run, i dont think its a good idea to tie guard to defensive stances in general, for this reason.
AoE threat? I mean if you never want to use any AoE and tank a single mob while the rest roam free all over the rest of the party, go for it. But I'll bet your friends wont appreciate it much.
space missions make a big difference. I ignored them unless I needed an exp boost to reach the recommended level for the next planet. And I skipped bonus series. I was 23 when I got to him. But I did every side quest excluding bonus series. Which is really about the way it's designed, you're the one who did something really weird and got way ahead of the leveling curve.
basic WoW type ui mods would completely solve ability management. The ui they have isn't very good for watching cooldowns, that's really the only issue there...
I believe the devs only said that the damage seemed about on par. And there's always going to be more or less complex classes too, to some extent that can be good since it gives different types of players options.
And I like the way taunts work in pvp in this game
Personally I always favor solutions that avoid homogenization. IE if sentinel 'huttball utility' came more from speed boosts and roots than knockbacks/pulls. It'd fit the flavor of the class better.