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.
See, I think that's the problem. Most currency is created due to the fact that people are already exchanging goods and services and they need a simpler way to do that beyond bartering and the like. The problem with BTC is that the currency came first and it's looking for goods and services to be used on, and to be honest, I don't see that getting any better. As others have said, due to the wild fluctuations, what producer of goods or services is going to want to deal with? And while I'm sure BTC will and can always be used for some niche things, I don't know if it'll ever get any traction.
The fact that a single person *anywhere in the world can send an **unlimited sum of money to any person of their choice without any rules or regulations being imposed is probably the most valuable property that bitcoin has. The security of the network is in the construction process as we speak. As more time goes by I believe bitcoin will be considered a powerful form of storing/transferring wealth. Have you looked at what would happen to single bitcoins if the entire economy were to have a value of just 1 billion dollars? Half a trillion?
*internet connection required
**depends on the worth of bitcoins