Bug-free, polished UI and properly portable are the main things. Ultimately no one person decides though - it has to be a group decision if a game doesn't make the cut. Hopefully the developer of the game in question will understand the reasons and agree themselves (indeed, I foresee voluntary withdrawals from those that don't do enough development in time).
This is necessary, by the way. We need to push ourselves to maintain a quality standard, and the threat of exclusion is a good motivating factor. For the bundle itself to be successful it can't have any weak links. And with the open timescale of the Incubator there's no reason to rush a game for the first bundle.
