Right now I don't think it can scale much bigger for several reasons. Namely the lack of "true" decentralization (as others have mentioned), but also the absence of even the most basic protections afforded to consumers in relation to securing wallets and mitigating fraud. There's still a long way to go in achieving wide-scale adoption...but the founding principles are sound and I think we'll see something akin to a Cambrian Explosion of new wrap around services for Bitcoin in the next 2-3 years which will make it a really useful currency.