A couple of years ago I would have said mentorship, but now I feel that I have enough experience and access to information to pull it off without one (not that I don't need it at all). Still, for novice entrepreneurs mentorship is crucial, if it's good, of course.
And now I'd say the Demo Day packed with investors and the real possibility of closing round A on the spot is the main attraction.
That being said, perhaps even more importantly for me at this point is a help with building a team. I know it's well outside of the accelerators' comfort zone - they usually accept only ready-made teams, in fact the team quality is probably the main criteria for admission. But what if a solo founder comes to you with a product prototype which already has users and revenues, i.e. a validated concept and business model. Would you accept such a founder and help him/her to find co-founder(s)? That might be a great help, I think.