Since bitcoin is still in its infancy, I would say almost everyone.  What bitcoin needs today to start on the path to becoming mainstream are merchants who will accept bitcoin.  Without merchants, it will never become mainstream.

But the biggest problem for that is bitcoin to fiat exchange rate.  It fluctuates so much that businesses would have to change their prices too often for their liking, so the merchants that accept it now are probably also speculators banking on the fact that the bitcoins they make today are going to be worth a lot more next year.  That is why you see service-oriented merchants accepting bitcoins more than product-oriented merchants.  Their fiat costs are low, and so they are essentially working for free hoping that in a few years, they would be making 10x what they could have charged in fiat today (speculation merchants).