I have a fic rec here that's of extraordinary Destiel fics (IMHO). 99% of them are long fics, and 99% of them AU.

Astrozoot's Completely Subjective Fic Rec List (http://astrozoot.livejournal.com/1692.html)

I can highly recommend:
And This Is Enough (http://cz-graphics.livejournal.com/9315.html) (bottom!Cas) The life of Castiel Novak, 27, animator, is kind of boring, his only highlight the sleepless nights when he's working on the animation of his celebrity crush, actor and heartthrob Dean Winchester. He starts reconsidering the monotony of his life, when he actually meets the charming man in person and they start an affair, despite Dean already being engaged...

Have Love, Will Travel (http://squeemonster.livejournal.com/54605.html) (bottom!Cas) Castiel Novak is a reclusive writer with a childhood so tragic it's left him terrified to leave his home—until his overbearing brother, Gabriel, drags him out for a night on the town full of booze and strip clubs, and he encounters Dean Winchester, a mesmerizing and mysterious stripper with secrets of his own. Both men find themselves inexplicably drawn to each other, and soon Dean's private dances for Castiel become much more, as both men confess their troubles and find solace in each other's company. But neither can seem to find the courage to take their relationship further than the intimacy of the club's VIP Room—and just when Dean's own brother gives him the excuse he needs to finally admit his feelings, Dean discovers something that brings it all crumbling down. Will they find a way past their demons and their trust issues, and back to each other?

Play It All Night Long (http://janie-tangerine.livejournal.com/290160.html) (can't remember who bottoms, but I would guess Cas does by the storyline) The rom-com-ish one where Dean hosts a late night radio show, Castiel is a regular listener of his who starts calling one day and ends up calling more often than not and Dean finds himself liking it. This, until one day Castiel calls for not exactly petty reasons (just before Dean's brother Sam is visiting with his girlfriend for spring break) and things get very, very crowdy at his place. He also doesn't know it's just the beginning of it. Also features Gabriel, Chuck, Andy, the Roadhouse crew and a huge amount of music quoted. Especially Bob Dylan.

Waves (http://archiveofourown.org/works/292406/chapters/467759) (bottom!Cas... I think) Dean Winchester is the average guy: football, college, kid brother, nice car, girls and beer; his life is black and white, that is until he meets Castiel Collins: pretentious, slutty, sweater-wearing genius, who won't even take the time to look up at him from his obscure novel while he insults him. And then everything is shades of gray and Dean is drowning.