The season with so much potential.
I hadn't realized so many different things had happened in this one season until I went back to review it: Soulless Sam, yes, and the time it took to figure out What's Wrong With Sam This Time, along with the not-fully-explored ethical dilemma of whether Dean should have shoved the soul back into him when Soulless didn't want it. Dean's wonderful deal with Death and the lesson he "learned" from it (which will never, ever apply where Sam is concerned). Meeting the rest of the Campbell family. The sub-plot with the Alphas and trying to get into Purgatory and then going after the Mother of All, including Dean's brilliant use of his own blood to hold the phoenix ash. A temporary truce/deal with Meg, which actually worked out that time. And finally, Castiel's machinations with and without Crowley, which did catch me by surprise when all was revealed.
Some really good episodes along the way: "Frontierland" remains one of my all-time favorites (though I still want an AU where they can't get back to the present for whatever reason and are forced to actually live in the Old West). The Brotherly Hug(tm) when Sam finally wakes up in "Like a Virgin" made me so, so happy. "My Heart Will Go On" had some really nice touches in the AU 'verse (like the travel agent's posters for Cuba) and an interesting "villain" in Atropos (who is probably still out there and still pissed at the Winchesters for interfering so often. Just sayin'.) And, of course, "The Man Who Knew Too Much" and Sam's insistence that he won't leave his brother alone out there was wonderful.
It's interesting how in the season after the one where the brothers were shown to be the product of decades of manipulation and interference--and still managed to choose their own path--that they were more powerless in this season than probably any other. Once we knew Cas and Crowley were working together, we understood it, but there was a whole of standing around and watching other people do things and trying to figure out why: the Campbells hunting the Alphas, Dean suspecting something was off with Sam and then not knowing how to fix it, Dean suspecting something was off with Cas and then not being able to fix it, Lisa and Ben being used against Dean, and the fabric of reality itself being radically changed with the Titanic AU and "The French Mistake," with the brothers in both cases being temporarily trapped in a different 'verse as a sideline to the real action. Maybe that's why I enjoyed TMWKTM so much--Sam got to make an active decision to reintegrate himself and then took the drastic step of stabbing Cas. (Even if, at the very end, they were left powerless in front of one of the most powerful beings they'd ever encountered.) It's an interesting contrast, of course, to how the next season ends...