(Installation instructions here)
On 2.3.6, there was an issue with screen responsiveness while charging, especially on the lock screen. That issue no longer exists.
Backgrounds to menus/lists are now black by default, rather than white. This is much easier on the eyes and better for battery life.
Ability to use cool new third party launchers like Apex and Nova.
Face Unlock. Very cool, and fast.
Built in screenshot feature (Power + Volume Down). Works great.
Scrolling is much smoother...almost like butter. This is likely due to ICS' hardware acceleration.
Real time voice dictation is awesome and unbelievably accurate.
More bloatware. Apps like Caller Name ID, Google+, and Video Calling are now included and uninstallable (without root).
Contacts tab removed from Dialer.
WiFi Tether currently does not work. Some people have gotten the stock hotspot app to work with the SQLite entitlement check hack, but I haven't.
Face Unlock. (Yes, it was listed as a pro too.) The downside is that it is unsecure. A photo of you will unlock it.
Notification bar icons for apps (on the left side of the screen) are small and darkened, which makes them difficult to see.
Built in screenshot feature has shutter sound that cannot be disabled, even in silent mode.
"Charge only" mode while connected to PC via USB has been removed.