Dropbox is a cloud service that offers and sells storage. BTSync doesn't store anything. You maintain the storage on both ends. The beauty is that there is no 5GB or 50GB or 100GB limit. You can sync as much as you want (or up to what your wallet will support in hard drives). One example is that you sync your laptop folders with your desktop folders directly. There is no middleman so-to-speak. Another example would be if you had a NAS (Synology, QNAP, etc). You could sync your folders to that. In effect, your NAS would be the 'cloud' that Dropbox sells you. Except that you'd have 4 or 8 or 20TB of storage space. And you own and control it all. Hope that answers your question. If you're just looking for a feature set comparison, I can't really help you out there. I think Dropbox makes its API open. So you'll be seeing a lot of 'Sync to Dropbox' options in the future. Think: take a picture on iPhone > send to Facebook > send to Flickr > send to Dropbox. Stuff like that... Personally, I think of BTSync as more of heavy lifting of files. Whereas Dropbox has more application tie in.
