How can I stop F8 key from bringing up Mission Control?
I ended up with this problem, too, after upgrading from Snow Leopard to Lion. I really needed my F8 key working as a standard function key for my job. I also really didn't want to delete my entire symbolichotkeys plist, though, so I did some digging.

If anyone else is having this issue still, you can edit your com.apple.symbolichotkeys.plist (located in Library/Preferences) with a basic text editor. Once you're in there just do a search for the number 75. Keys 75 and 76 (76 is the slow animation for when you hold down shift) are the ones for F8 activating mission control. You can just delete those two entries and your F8 key will become just another normal F-Key again. F9 will still work for mission control and you won't lose any other hotkey settings either.

As usual, always make a backup of these files when you're messing with them.

Good luck to anyone else with the issue, and thanks to those here who'd already provided some input and answers.