That's a fair assessment, but that's because games based popular genres tend to be fun, even if they aren't unique. Those genres are popular for a reason. Eventually players will stop finding them fun, and these popular genres will phase out, at which point they will stop attracting sponsorships. But I don't think that any sponsor chooses to bid purely on polish. If the game is fun (and fun can sometimes be achieved in a "sellout" manner by copying or cloning existing genres), it still needs to be polished to be successful.
Of course, I think that serious hits do have to be unique, as opposed to direct clones of existing games/genres, but they still must be polished, if only to convince players and sponsors to continue playing past the menu screen. Polish is a good indicator of how much time and effort a developer put into a Flash game. If the polish sucks, the gameplay likely does too.
Hmmm. I don't think sponsors really care about originality. They do care about fun but the kind of fun that is appealing to kids and parents. Original game may be received well on Kongregate and Newgrounds but if portal owners don't pick it, it's a fail from sponsor's point of view.
I recognize two ways to evaluate if the game is fun:
a) it is fun to sponsor
b) it is a combo of great polish, popular genre and adequate theme
As far as my experience goes, I think they try to avoid a) because what's just a personal fun is most likely not going to go viral.
With that in mind, I think sponsors are resorting to b) which is judgment based on combo of polish, genre and theme. It is another level of safety because portals that host games are thinking in the same way.
I have to say that Newgrounds and Kongregate are probably the most mature flash audiences out there. Other portals are driven by 10yr old kids playing with their parents. That said, majority of portals will only pick the game if it's casual and easy to play, avoiding original, unique games that are genuinely fun to mature audiences. That's why a game can do bad on Newgrounds and Kongregate but still get more than 30 millions of plays in one year.