No to avatars.
No to karma stuff.
No to sorting comments by popularity (people will start trying to game the system at the detriment to the quality of the posts).
IMHO this kind of stuff will just detract from people engaging in putting forward serious comments and engaging in interesting discussions. I really like the fact that certain forums I visit do not have avatars and very few people have bothered to make sigs (of which the ones that do usually put useful info/links in).
Nesting - you shouldn't have brought in the silly curved arrow to indicate someone's post is a reply to someone elses, with the vertical grey bar it was instantly obvious it was a reply, and if you bring it back you can make it so two vertical grey bars mean a reply to a reply, and perhaps stretch it to 3 for a reply to a reply to a reply.