I'll be honest right off the bat--I'm just not a fan of the "wet mane" look on anypony; pictures, plushies, whatever. It's not my thing.
But we'll put that aside and I'll critique the rest
The Outfit: I have seen another outfit like this one already, but you took it to another level, adding the ruffle and the bow tie, not to mention the detailing on the socks. This outfit is hotter. And though some may not approve of putting ponies in such suggestive costumes, I believe that it stands to reason that Rarity would make apparel other than gowns.
The body: well sewn and well proportioned. The mane and tail are well-done for the body (although I would prefer the traditional Rarity hairstyle). The ears and the horn are in the right place and also well-proportioned to the body.
The eyes and the cutie mark, as always, are wonderful--I want to know how you do this.
I wish that Hasbro would consider hiring some of the fan talent (like yourself) to design and run production on plushies for the franchise--you always make stuff that I could see myself or other fans buying. It seems a shame that I can't just go to Hot Topic or Spencer's or Target and pick something like this up. Very nice job on this one.