Agreed. The "Top Contributors, 30 Days:" would definitely make a lot more sense if it was based on your suggested "karma from recent posts/comments" instead - right now it overlaps with the "all time" list except for gwern.
I definitely agree with this part. (Even if it means I can expect to spend less time in the 30 days top!)
Is the karma shown that accumulated from posts and comments that were made within the past 30 days, or from all votes on all posts within the past 30 days, regardless of the age of the post or comment?