Someone please tell me if StarTrek.com is considered canon. I thought only televised, movies, and a couple of novels are considered canon. If that is the case, let me ignore the info from StarTrek.com and get directly to the relevant dialogue:
Kevin Uxbridge wrote: I am a Douwd... an immortal being of disguises and false surroundings. I have lived in this galaxy for many thousands of years....
He also says:
Kevin Uxbridge wrote: I could have destroyed them [The Husnock] with a mere thought, but this I did not do...I refused to for the same reason I refused to stop the Enterprise. I will not kill!
We also see him as a ball of light toward the end of the episode, but that doesn't necessarily signify omnipotence; also the ability to kill a race with a thought doesn't necessarily mean he's all powerful. Picard describes him as...
Captain Picard wrote: We leave behind a being of extraordinary power...
Coupled with his ability to generate illusions and fuck with Troi's mind, I would say he was more of a super-telepath who could even fool starship sensors with his powers. There was no evidence to suggest that he could manipulate matter. If he could have, why not manipulate the matter of the Husnock ship(s) in a non-lethal way?
And just because a being claims they are immortal doesn't mean they are. "Immortal" beings die in science fiction all the time. People mentioned deaths in the Q continuum earlier, and I'm sure the Dowd may be pretty tough (he survived the Husnock equivalent of a Base Delta Zero) but nothing lives forever. Nothing.
On the flip side of the coin Q has clearly demonstrated much more power than Kevin Uxbridge. I have to say Q would kill him, and if Uxbridge was truly immortal, then Q would kill him again and again until he got bored of it. After that, who knows?