Well, it's distributed. Benchmarks indicate that it's considerably faster (given its distributed nature, operations like diffs and logs are all local so of course it's blazingly faster in this case), and working folders are smaller (which still blows my mind).
When you're working on subversion, or any other client/server revision control system, you essentially create working copies on your machine by checking-out revisions. This represents a snapshot in time of what the repository looks like. You update your working copy via updates, and you update the repository via commits.
With a distributed version control, you don't have a snapshot, but rather the entire codebase. Wanna do a diff with a 3 month old version? No problem, the 3 month old version is still on your computer. This doesn't only mean things are way faster, but if you're disconnected from your central server, you can still do many of the operations you're used to. In other words, you don't just have a snapshot of a given revision, but the entire codebase.
You'd think that Git would take up a bunch of space on your harddrive, but from a couple benchmarks I've seen, it actually takes less. Don't ask me how. I mean, it was built by Linus, he knows a thing or two about filesystems I guess.