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.
This is harder than it sounds. As a business trying to deal in bitcoin I have found it very difficult. The reasons I have found it difficult is people for some reason think that I should be practically giving away the hardware I sell when I do it in bitcoin, who knows why that is but most times people want to offer my 2/3rds what I would sell something in USD for. Also, because there is a big problem with trust transactions have been taking me longer to close.
I'm going to keep trying for now to sell things in bitcoin but I don't know for how much longer because it has started being a bit of a pain in the neck.
It is because pretty much everyone who has BTC wants it more than everything else on offer. You have to be fairly enticing to get it from people who have declined everything else.