I highly recommend The wings were in the waypart 1 (http://community.livejournal.com/deancastiel/1903587.html#cutid1) part 2 (http://community.livejournal.com/deancastiel/1907250.html#cutid1).
summary: Interspecies dating is complicated. The slow build of a not so normal relationship.

It's very AU and the author did an amazing job building this world!

Brilliant Dean/Castiel au "I want you all to myself": http://melfice.livejournal.com/263320.html

:-)

Also you would probably enjoy:

The request (http://cloudy-fic.livejournal.com/tag/request)
Summary: When Sam Winchester prays for his brother, Castiel is finally sent on his very first assignment. But what should be a simple love match turns into much more and Castiel finds himself risking everything to ensure the happiness of his extremely frustrating charge.

Castiel's Star (http://www.fanfiction.net/s/5487087/1/Castiels_Star)
Summary: set in regular world, not spn world. Dean and Castiel were childhood sweethearts until Dean fled their small town and left Castiel behind.Now ten years later Dean has come back to set things right.

Veterans of the psychic war (http://community.livejournal.com/psychicwar/3181.html)
Awesome AU set in the future. Psions are drafted to help shielding the outposts of humanity against outside (alien) attackers. For most Psychics, a bonded partner, their guardian, is a given, mostly family. Not always though, and Jared, young and still hopeful, wouldn't have dreamed of finding one. Least of all the antisocial Jen. Awesome, action-packed, would make a great movie.

Feeling is Freedom by arlad: http://arlad.livejournal.com/58214.html

Dean/Castiel AU
Asunder (http://archiveofourown.org/works/62115?view_adult=true) by rageprufrock

Peanut Butter Pumpkin Wedding Cake (http://sparseparsley.livejournal.com/8666.html) by sparseparsley

I Think I Know Where You Belong (http://twentysomething.dreamwidth.org/2373.html) by lillianloop

Ru_salki99 has some good ones - including rentboy!Dean, and russian mob!Cas, and a really sweat one where Dean plays depression era baseball.

http://community.livejournal.com/the_readingroom/133697.html