I'm not saying that Supernatural has never retconned something - actually, I recall a discussion recently (with claudiapriscus?) where someone finally was able to point out something to me that was ACTUALLY a retcon. But most of what people are calling retcons ARE NOT RETCONS.
There is a HUGE difference between CHANGING previously established canon, and ADDING to the canon that we ASSUMED we knew.
Brady is not a retcon. Did Sam ever give us a list of his college friends? Did we ever hear the story of how he met Jess? No...therefore, writing Brady into the show in S5 isn't a retcon, it it GIVING US INFORMATION WE DIDN'T HAVE BEFORE!
The Amulet as God-EMF is also not a retcon: Did we ever learn why Bobby told Sam that the amulet was "real special"? No. Now we know though!
The Trickster actually being the arch-angel Gabriel is not a retcon. It is the arch-angel Gabriel being damn good at pretending to be a Trickster!
Sometimes I think our knowledge of the way the writers work is detrimental to the way people interpret the show. Because we KNOW that Kripke didn't set out to have angels in the show - we know that when the Trickster was first introduced they didn't plan to make him Gabriel. But that doesn't make the the Gabester a retcon. It just means that the writers decided to add in the information after his initial introduction, and were able to do it in a way where it actually made a little bit of sense (because, seriously, what was it to the Trickster that the Winchesters kept sacrificing themselves for each other?)
Now if there had been a line where Sam and Dean did some sort of crazy Angel-test on the Trickster for no reason back in S2, and Dean announced that the Trickster was definitely not secretly an arch-angel, then Gabriel would be a retcon.
Right now, the only "retcon" I'll give lea-way on is Chuck=God (which is heavily implied but not specifically stated)...and that's JUST because they met him in S4 when Dean still had the God-EMF Amulet and it never "grew hot" - at least not to the extent that Dean noticed.
Sigh...sorry, I get so annoyed at inconsequential things sometimes.
Also, if any of you are following the twitpics of Asylum Europe Con this weekend: I like Jim's shirt, but the grammar is incorrect and it's driving me crazy.