1. Upvoting costs an amount of karma points proportional to the number of points the article has at that time. Hence, it costs more to upvote an article on the home page than it costs to upvote an article in the New section.
2. If I choose to spend karma by voting on an article and it starts to rise, I get proportional "returns" on my karma.
This incentivizes me to upvote New articles I think HN will like. But then again if I lose all my karma points, I might need a bailout.