FM Radio doesn't work - CM9 does not have FM Radio support for any device - the CM7 radio support would need to be forward-ported and then made compatible with GS2 without breaking other devices
GLONASS satellites do not report as locking in GPS Status and similar programs - this is most likely a display bug and they are locking, but needs more investigation
USB OTG doesn't work
MHL doesn't work
/emmc and /sdcard are swapped compared to all other CM devices - This is why the "internal" SD card in CWM is actually the external
Control of dualmic (Samsung Noise Reduction) in the phone UI is not implemented - Info on controlling this can be found at http://forum.xda-developers.com/showpost.php?p=23088636&postcount=325 - someone who is good with Java UI work could implement this control in the Phone app. (I suck at UI stuff - Entropy512)
Volume does not always restore properly when a notification comes and headphones are plugged in.