It's a good idea... But it would need some tweaks.
I think one big problem may arise with the original design. And it is one that is plaguing sites like IMDB right now. People may vote based on what they think AND what it's current vote is at. Aka: I didn't like this post, I'd vote it a 4. However, it's voted a 9! I will vote one to drop it down closer to my vote. (The Logic is clear... And dumb) This problem leads to a ton of unnecessary 1s and 10s. (BTW, this post is a 7-8 for me, haha not 4!)
A solution for this is hiding the total from the voters, and only showing it to the author.
Another, less obvious and less important, problem may be the difference in thought; so what I think is a 6, you think is an 8. Although it's a problem, I doubt it would be a large one... Just good to enlighten the few that didn't already think of this one. (It's really small... Although it could be important, most likely it will not be.)
(Edit: Yes, I wrote this as if the slider didn't already exist... Haha, not yet for me at least. Hope my "improvements" help... Possibly.. Thanks)