First I want to say I agree that the JKG threat generation is a little on the weak side. We don't have much in the way of sustainable AoE like the OP mentioned and our damage is so low that from just attacking it's hard to keep threat on multiple targets. I'm basically forced to use my taunt abilities as soon as possible in real fights.
Now, I'm not saying that the threat generation is seriously under-powered for two reasons. First, against single-targets I can hold aggro just fine. In fact, the easiest fights are against a solo champion. So at least we do that fine.
Second, SWTOR isn't quite balanced the same way as other MMOs (I won't use the W word). As other posters mentioned it's actually not critical for the survival of the group for my JKG to hold all aggro. Basically, I just have to lock down the heavy hitters. The rest of my group can use their long stuns and/or just wipe out the trash mobs. I find that if my group can survive the initial rush of combat (specifically when there's multiple heroic elites) they the rest is a breeze. I've come to accept JKG does not equal Paladin or Warrior tanks from the game-that-won't-be-mentioned
Now, all that said, I feel like I'm tanking the hard way. My group does fine and but I can't help but feel like it's because they're carrying me a little. I've made a Vanguard and have started levelling him up to the level of my main group. So far Vanguard is easier, but I haven't caught up to my JKG yet so maybe Vanguard get harder in the later levels.
If there's anyone out that that can give first hand comparison between the tanking ability of a level 50 JKG and Van, I would very much like to hear about it.