Internet Sharing Issues (WiFi to Ethernet)
I don't have a definitive answer, but I've got some ideas to start, and perhaps we can narrow it down as you try some things and provide more information.


For starters, check the lock status of the Sharing Preferences. The lock icon in the bottom left corner should be open when you make changes. I've run into some problems where the interface would allow me to make changes, but they wouldn't save because it was locked.

If it is open, set up sharing as you would, then lock it after. See if that makes the configuration stick.
Try removing /Library/Preferences/SystemConfiguration/com.apple.nat.plist (you may want to make a backup copy just in case). Then try re-enabling Internet Sharing (which should regenerate that plist file). It's possible something got corrupted in the configuration file.