As I always do when someone starts off on this flight of mythology, allow me to suggest that you read Clay Shirky's "A Group Is Its Own Worst Enemy":

http://www.shirky.com/writings/group_enemy.html

You may learn some things about these cycles of "I was on Site X and it was great, then went to crap, then I went to Site Y, then it went to crap..." which will surprise you, and which -- at least in my reading and based on my experience of spending the last decade building community-oriented sites -- hint that reddit's doing way, way better than HN.