The convergent linear rewards curve taking from the low value posts will hurt new users. We can hope more curation will help out with that but there's no guarantee.
if you want to reward someone for a good comment on your post what will be the minimum of SP for 100% vote to jump over dust vote?
That's a pretty good question, which I have not seen raised before. Someone should probably work that one out if single votes will just become dust for most users.
Downvoting will be interesting. Rewarding downvotes I think would go very badly. Offering a few downvotes a day wouldn't add too much fuel to a downvote war but could be sufficient for better use of downvotes. If downvotes could only be taken from a separate smaller downvote pool and not at all from an upvote pool, we could see less abuse of downvotes as each user would have less overall downvotes even though they are free.