I'd been on a $20/month plan with Dropbox for a few years now. I recently cancelled down to the free tier because...
* I didn't want to pay more to store/sync even more data.
* All my apps that add great convenience by leveraging Dropbox for sync will work fine and consume just a fraction of the free tier space.
This is actually the "killer" feature right now. If I had to pay to keep this functionality, I'd probably pony right up, but it would be tough to start demanding payment for that.
* Dropbox is actually pretty slow.
Sure, I'm just one person, but their percentage of paying customers has always been small. Just a few years ago, there weren't many if any viable alternatives - now there's a whole slew of them and Dropbox hasn't changed much if any in that span.
Also, since it seems to come up so often. I've seen nothing in the way of moves by or significant interest in Dropbox in the enterprise. Meanwhile, Microsoft is pushing SkyDrive, Google has Drive and Box is stating that they're fully enterprise focused.