Ever tried committing a 100meg file to a Git repository? It'll peg your CPU to 100% for 45 seconds. I like the idea, but Git isn't a great general blob-store for this kind of project.
Media storage is, I believe, a common Dropbox use case. And Dropbox's transparent uploads, which seem to automatically detect and throttle when I am using bandwidth, are a great feature that I haven't seen in any open-source competitor.
Also, one of my favorite applications of Dropbox is having all of our important documents in one central location (whether this is a wise move or not... ). This makes accessing documents on the fly from my iPhone a breeze. Having media files in there as well, would, I think destroy my antique iPhone 3G!
You are right on the money about the background uploading though.
The iPhone app doesn't sync everything - it will download (and separately, cache) only the files you request.
the 'append only' nature of git makes it unsuitable for this application imo. a sliding expiration/history window size depending on avilable resources is ideal
And then, besides all that, the major problem with committing blobs to git is that git will store the blob once for each (potentially small) revision that you make. That's not space efficient, but the git community knows about it, and is actively trying to fix it.