Actually, what made BitTorrent popular was not the open source and cross platform compatibility, although those probably helped to some extent (by building goodwill if nothing else).

What really made BitTorrent big was the transparency and ease of use. You clicked on a link on a webpage, and you got your file. This opened it up to a much larger audience than those who can bother to download, setup and learn a whole new program just to get their file.

It succeeded because it was minimalist - if you put all the complexity back in, it won't be better. The warez kiddies will use it, but they'll use anything that gets them their files. Normal people won't be so easily impressed.