If you genuinely want to make something people want, observe that file sharing clients like eMule and uTorrent have tens or even hundreds of millions of downloads. They actually make even the biggest Web 2.0 services like Flickr look small in comparison.
People will put up with bad interfaces and installing desktop software if it gets them what they want in a new, faster way. This has been proved several times since Napster arrived.
So why not make a better BitTorrent or something?
Oh, and I know Bram Cohen's company took money, and there are online services for transferring large files around, but as far as I know that's about it.
If you think about most content out how much is owned by the creators?
I say you should make the next bittorrent, if it becomes successful you'll have armies of lawyers saying your the pirate even though you just created a way to file transfer.