The quixotic infidel had a very good visualization for describing the canonical status of various books by various entities in early Christianity.
Michael from Pisteuomen is in a discussion with the author of the blog on the authorship of Hebrews. I happen to agree with Quixie; Paul did not write Hebrews.
I did my thesis on the pneumatology of Hebrews at DTS. My goal wasn't to deal with the authorship question, but the issue came up anyway. After looking at the rather (comparatively) weak pneumatology in Hebrews, I was (and still am) convinced that the epistle/sermon did not come from Paul. The Spirit as sanctifying influence is a major category for Paul but one that does not show up at all in Hebrews. That would not be odd except for the fact that sanctification is a major theme in Hebrews; ergo, thematically, I don't see any way that Paul could have written that thing.
Granted, that's probably not enough of an evidence by itself to reject Pauline authorship, but I do think it merits strong consideration. That plus the many typical reasons basically has me convinced.