> 2. You can only sync a single folder, which is really limiting

That is easily worked around as Dropbox follows symlinks.

E.g on my desktop ~/Dropbox is on a small SSD so on that machine ~/Dropbox/Photos is a symlink to a folder on my large capacity HDD.

> 4.

Perhaps you already aware of the symlink solution :) It works for me, but is a little hacky. And you need to make sure dropbox is not running when moving and soft linking folders.