Aside from freshly being the most powerful being in the universe? The fact that he ripped an armada apart (not to mention it was someone who was just gleaning his energies from him a little bit), while having a non-focus fit? The only one we've seen do ANYTHING is a guy who was stealing a fraction of his power, until Kyle came along and fixed it. Not to mention the Guardians seem to believe his power is on their level.
Plus, doing something easier than a Green Lantern is one, a rather large task, as Green Lanterns are pretty much the big boys of the universe, and two, he hasn't had much to do with that power yet. But everybody, and that means everybody, seems to be worried about it.