I agree. At least on the Mac client, there's already an "Advanced" preference pane. Why not create an option there for the user to manage their own encryption key, and possibly what folders it applies to? (For example, they could exempt shared and public folders but encrypt everything else.)

You type a key in some proprietary software. Anything beyond that is only based on trusting the software.

Then open source the crypto portion. Problem solved.

You still have to trust them that they do not transmit your keys elsewhere and that they really do use the same crypto code thy open-sourced.

I am not saying that you can't trust Dropbox. Quite on the contrary. But if you give your key to some unknown entity, you have to trust them. You can choose to trust their claims about what they do with it, but there is ultimately no guarantee.

Beauty of asymmetric crypto, you don't have to trust them as soon as you can trust the client software (that it doesn't do something nasty with your private key/pass). And having an open source client can actually provide that trust.

Or you can use other products if you don't like their decisions.

Or complain enough such that you, and all your friends, stop using the service. It's an even more effective way to convince the company to change.

I'm already doing that.

Indeed, encrypting something that's already encrypted isn't a bad thing