This isn't really an answer to the question of influence, but it gives you a start on the question. There's the list of the top 100 people ranked by karma - that doesn't mean they're influential, of course.
Aight: How about "The capacity to have an effect on the character, development, or behavior of [the collective mind of the people who read HN] ...something like that.
Check PG's profile - he has 124701 (at the time of writing) and would be significantly off the top. He has deliberately removed himself from the listing.