if you've finished the sixth book and want to discuss your theories and swap condolences... well, i'm afraid i'm the wrong person to talk to. i haven't been able to muster the time commitment to really get attached to Mr. Potter and his buddies.
however, my family is nothing if not totally into it. i have picked up several of their theories, but perhaps the best thought out and most probable is my unlce's theory about who the real villian will be at the end of the series. it almost makes me want to read all six of those big books. he's such a fan that he has an entire blog post about his predictions. if i'm interested in them, they are sure to be a hit with a real harry potter enthusiast. but be careful! there are spoilers!
EDIT: the link has been taken down.