These new slim controllers suck. They are smaller than the original Xbox 360 controllers and are not as grippy on the underside where your ring and pinkie fingers hold the controller. It is a shiny, slippery plastic now instead of the the more rubbery or matte feel to the old ones. The thumbsticks are also also closer together, which threw off my aim in BFBC2 and MW2.
After I got my 360 slim I actually bought the Wireless 360 controller for Windows so I could have the old controller style rather than the new one (and so I could get the wireless dongle for PC). New people to the 360 probably wont even notice the change but if you have been using an older controller for awhile and play a lot of shooters you will notice instantly.