In particular, Time Machine makes large use of hard links to directories, which not many filesystems support. With HFS+ Apple can be sure that support is always there.
Actually I think you'll find it makes use of hard links to files. Its basically a reimplementation of rdiff-backup, or it might be the other way round. I can assure you that no directories get hard linked, and I'm sure someone will furnish the obligatory xkcd.
Edit -- I stand corrected! It does in fact link folders as well.
Also:
http://xkcd.com/981/
It hard-links directories, which is non-standard but supported by HFS+. It's kind of crazy to do in general, but in this specific use case it's a great idea.
Edit -- I stand corrected! It does in fact link folders as well. Also: http://xkcd.com/981/