When people say Dropbox is missing encryption, they mean encrypting files before they are uploaded so not even Dropbox could look at the files - end to end, so to speak.
They do not mean that Dropbox is insecure. Dropbox is secure.
The problem with end to end is that it makes sharing or public files unfeasible, and those are important parts of Dropbox.
Have you considered making encryption available on a per-folder basis? The encryption would be done on the client side so it wouldn't incur any additional computational expense for your servers, but would still provide the peace of mind that many look for with TrueCrypt containers. This could be even be done in a way to allow the user to choose his implementation.
I do this currently, but I wish there was a more elegant solution.
As far as I can tell from my own usage, the only portion that is uploaded is the changed portion. I have a 1GB container currently synced and when I change files in that container, I can assure you that it doesn't upload 1GB of information. It appears to only transfer the delta.
