I love the Galaxy S2, It is a really well built piece of kit. There are, however, a few little annoyances with it, some of which can be fixed if you don't mind getting your hands dirty.
Here's what you need to do to get going.
This will give you writeable access to the file system, now you can fix those annoyances.
If you know of any other tweaks, let me know. I'll be keeping this list updated.
There's also lots of power user features built in, so check them out too.