1) You can submit one link a day. Additional submits cost karma.
2) Costs karma to reply to any comment. Top level comments seem to already filter okay. If you get downvotes on the comment you made the karma cost is a multiple of that.
I also think some things would help in general:
1) Title/Domain Regex - Allow me to specify a regex to exclude things from the frontpage. For example /Apple|iPad|techcrunch/.
2) You have to have 10 or 20 karma to do anything besides top thread comments. It would be easy to get that with a little effort, but it would pretty much eliminate all the spam and low hanging crap.
3) Have a option to (turned on by default) to collapse comments using the common +/- interface and display the total score for that thread. I think then you would be able to focus and find the good threads quickly. Coming into this discussion 5 hrs after the fact like I am doing is where this is really needed.
4) This is a big one, but I will throw it out there. Create an API. With that I think a LOT of smart people (instead of a few) could play with all of this and maybe find somethings that no one here is currently thinking of.