I love my S2, but I *do* have a couple quibbles with the design:
Answering an incoming call: with my hands full and after digging for my phone, it's a pain to have to catch the stupid little red window and pull it across the screen in order to answer the call. I want to just push a button to answer it.
Getting back to "home": most of the time, pressing the Home Key is all that's needed, but sometimes it's not. Then you have to keep pushing the back arrow.
Security: if you set a PIN or password for access, it requires you to use it every time the screen times out. I'd like to have one PIN for when I first turn it on, then another (that I can set separately) for when it's simply asleep due to inactivity. I've ended up using NO security because it's a pain to use this every time the phone goes to sleep.
Keypad: a pain to use. I often get the wrong characters, but more irritating is having to flip back and forth between the letters keyboard and the numbers/symbols/punctuation keyboard. A big time waster!
I did download the PDF user guide but haven't found it particularly useful.