Well, I'm pretty new to MMO's in general but I personally have found it to be not particularly difficult to level when I'm up doing the defensive tree. I honestly can't tell you how many times the defensive buffs have saved me from dying during any of the boss/champion/elite encounters in the game. It's really quite nice being so survivable. I've found that while you don't do as much damage as the other classes you can mix and match spacing out your defensive abilities along with your offensive abilities along with your debuffs to make so that during a fight you will consistently do more and more damage to your target while your target does less and less damage to you. At level 30, I can force leap in stun, sunder armor and then hilt strike for huge damage and an additional stun then hit them with other attacks to build focus during the stun, stasis, then sunder armor and then force push them away from me and then jump back in with another force leap to stun them and then start the cycle all over again. Granted it has taken a while to kill some units in this way but it's effective and they're taking huge damage all while not really being able to attack. That's really only one example but it's the thing I do most often. It even works quite well in pvp when I don't screw up or as long as my team is keeping my targets team at bay. Anyway, I'm fully enjoying the defense tree as a guardian in PvE and being full tank hasn't actually impeded my leveling process one bit. Hope this helps.