It's not that, Guardians are just really, really late bloomers.
They get their best skill at 50 at which point they pretty much paWn everything.
But devs have said it's the one class they are going to tinker with in the upcoming big patch. My guess is they reduce all the redundancy in their skills. That would help a lot. A AoE taunt would be nice too--or get aggro generation much sooner than they do.