I think the point limit is useful, so that a user doesn't just post one popular question and end up getting enough points to gain privileges quickly.
The guy who only ever posted one question (move combos) could end up being able to edit content on here after spending about 10 minutes on the site ;) I guess it prevents spammers a bit too.
I might raise the limit a little - currently it's equivalent to 20 upvotes. But on the other hand there are barely 10 posts on the entire site that have 20 upvotes.
And to answer your question - as far as I know it's just based on the final vote score, not individual upvotes and downvotes. So if it says +15, it's 150 points regardless of whether it's 15 upvotes or 20 upvotes and 5 downvotes.