Whether he is guilty of treason, in my mind, is dependent on two things:
First (and much the most important) is whether any material he passed on was actually damaging to the US military or citizens, resulting in loss of life, injury, or serious property damage; or if he had reasonable expectation that it might do so. So I would not include those embarrassing, dirty little diplomatic secrets that seemed to be most in the news when this was a fresh story. I am neither a lawyer nor an expert in the legal definition of treason, but if all he did was embarrass some high level officials, I’m not nearly as concerned, and I would rather he were charged with something less than treason.
Second is his motive. If he had done it for money, or notoriety, or to get back at his CO, or some other unhappy reason, no excuses, throw him in prison (unless, see above, in the case of actual damage, execution might be called for). If he did it in the (possibly misguided) belief that he was helping make a better world, then I am inclined to be slightly more lenient in the sentence, if not in the actual charge filed.
The bottom line, to me, is that no-one in his position should be deciding what classified information is passed to non-authorized persons, but there should be a very high standard to try, and possibly execute, someone for treason.
ETA: ok, I missed the above correction before I posted. No, I don’t consider him a traitor unless condition one above was met. Whether Wikileaks is the enemy is not the point; passing information to Wilileaks means that other people than them will also see it and possibly use it. The issue should be the seriousness of the information leaked, not the person to whom it was leaked. Even if it was leaked to a “friendly power”, it’s still potentially traitorous. There are legitimate reasons why we don’t share everything with our allies.