I think for one thing it would cut down on the phenonenom I've noticed here where slightly controversial comments are immediately downvoted, but which then are upvoted back to full visibility. There seems to be a dedicated core of HNers who zealously and frequently downvote any comment that's not directly related to hard-core technology issues (there may be other motives, I've not examined the patterns carefully). These quick dowvotes are often enough to cause some feelings of ill-will that persist through the rest of the exchange.

I think limiting the number of available downvotes would drastically reduce these quick downvotes, which would be a good thing.