I like the 'size' of it; big enough so that there are always new posts, but not too many.
You can post here without having your post drown in a sea of other posts within a minute, and you don't need to turn posting into a second job just to become a recognised face.
Obviously I also like the people and the discussions, but I'm not all that into keyboards, so there are plenty of other places I could hang out. The reason I like GH so much is because I can actually be helpful here, I can say stuff that matters.
Try the same on reddit or a large forum and your post would either be drowned out or someone else would've said the same anyway, only more informative.