Well.. First of all:
Say you are working on a feature that took 10 hours to implement and test. Others may have committed other code to the repository in the meantime. You're code is based on an outdated revision of the repository. If you try to commit, You will have to merge alot of stuff which is very tedious. The actual conflicts may not even be related to the changes you made.
In Git, merging is very easy and fast and the system keeps track of what you merged and what not. Furthermore, it's decentralized, which means you can make as many "repositories" as you want, which makes sharing unfinished code with other developers easy (without having to commit it).
Subversion is slow. When you are working on a "larger" project and you want to retrieve the log file, It'll probably take about 20-30 seconds before you see it.
Subversion does not check the integrity of a repository, memory and harddrive failure may cause a repository to become corrupted. Git makes a hash of a repository and always checks against this hash before doing anything with it. If it becomes corrupt, It will let you know.

Disclaimer: I did not use Git yet, but I read and heard quite alot about it. I don't know anything about it's usability (But that should be okay when you use a nice frontend, like TortoiseGit). I have installed it though, and I will check it as soon as I have time.