The Guardian is designed around close combat, sustained damage and control. The Burning condition which comes pretty much for free with the Guardian is a particular example of sustained damage. I don’t know any other class that can apply Blinds (which are a great damage mitigation tool) on a regular basis. And Scepter is actually very good for ranged PvE, and it will also be better in PvP once dev’s will buff the projectile speed as already promised by John few days ago. And I am not even starting about the Staff, which is simply an amazing weapon.
Yes, Guardian has its limitations. But so does every other class (save for Ele ^^).
Thief can apply blind on command with dagger/pistol, limited only by initiative regen.