I like that Reddit's voting system is a lot more democratic than Digg's system where there were "power users". I think this also helped create the strong and, at the same time, large community.
This allows you to do things like filter results to only high scores, excluding 'funny'.
Slashdot's comment moderation system always seemed to work pretty well to me. The general editorial quality is what made me give up on them. (When I checked back recently, most submissions were incredibly blatant flamebait.)
IMO the biggest problem with Reddit is that too many people downvote well-supported arguments because it goes against their belief system which really stymies the conversation. I'd like to see downvotes haves some 'cost' associated with them so they are used sparingly and on content that really deserves to be downvoted.
There's always going to be a way for determined users to get around the limits, but if it can reduce the number of reflexive downvotes from most users it could still be a net win.
I don't agree with their policy that those who vote in a thread may not comment in that thread but I think a lot can still be learned from Slashdot's model.
No the biggest problem is that for the majority of reddit users that 'weak content' is actually what they come to reddit for. It's a systemic problem with the community. The comment system works fine.
The "flag" option would have to include a reason ("offensive", "off-topic", "spam")
The main problem with downvoting for me is that often people vote down out of disagreement, resulting in groupthink. In my opinion, downvotes should hide comments and punish commenters that don't contribute to the discussion. By making downvoting slightly harder, you could focus on the proper subset of comments that should be punished, like spam and insults.
(and by the way fix the UI problem of two arrows that are much too close to each other)
In all, this means downvotes are uncommon; usually, people try to fix the problem (editing or asking the writer to edit). However, if something is downvoted, it is extremely likely that that post is irrelevant or actively wrong.
There would be no reasonable way to force people to make a useful comment.
Redditors' reputation is depends on their actions, not points, badges, friends or any other "scoring" system. This makes for a way more dynamic and sustainable meritocracy than sites who's system tends to create an incumbent elite of power users.
The color of the post should only change when it hits a negative score. A post downvoted by only one person is a reflection of that person rather than the post. Also, since they start at 1 point anyhow, it would be a very elegant system.