They have a business model that requires payment for anything but the most trivial uses (which is great, they'll be profitable, but it is a definite brake on growth). Hard to do that soft of thing without charging for it, and as soon as the price goes over $0 the market is much smaller.
It's a much bigger decision to pay at all than it is to pay $10 or $20.
The consumer space is growing (more devices and users) and the enterprise space is wide open. I can see Dropbox going after Sharepoint et al.
You're right, it might not be enough to be the next google but they might come close.
You're basically setting yourself up to have a 'gmail' pulled on your 'hotmail'.
PS: Nobody things the best way to beat gmail is to offer 1 TB email accounts. Email fell behind the HW curve but as long as Dropbox stays reasonably close to actual costs nobody is going to dramatically undercut them.
I haven't converted to a pay using yet, but I'm at 50% of my 3.5GB. When I hit my space limit, I will likely switch.
Last week I had to send a PDF to one of my students (an undergrad philosophy major) and suggested dropbox, and she already had a public folder set up.
Software companies in the valley have gotten along just fine charging for services and software with only giving users a token free trial. Microsoft and Oracle are doing just fine.
If you want to look at the big players on the web, there are far more public companies charging for services then there are whose main product is free. Akamai, Sales Force, Amazon, Ebay, NetFlix, Omniture (now adobe) I could go on and on.
When talking about public companies who give their primary service away for free, I can only think of 4examples with market caps over a billion: Google, Yahoo, IACI, and sometimes Monster.
There is a lot of hype about free, thanks especially to Chris Anderson's highly accessible book and influence, but if you look at the results and who really made it big (not 100 million dollar exit big, but multi billion dollar market cap big), starting with a business model where you charge your customers is the best way to achieve that.