Add downvoting to submissions as well as comments. Problems this will alleviate:

-users will have a way express that they feel a submission is egregiously off-topic or extremely redundant rather than leaving the off-repeated comment "This is Hacker News?" or "Here is a script to hide links with the word iPad" etc.

-Will improve the quality of submissions and prevent people from karma-fishing with linkbait titles on articles of little value.

-reduce multiple copies of the same story from sitting on the front page