I want to add my voice of dissapointment / disapproval to this rapidly growing list!
Airplay mirroring was also one of my primary reasons for moving to Mountain Lion (which also required me to shell out to upgrade to a newer version of Parallels because there are a few Windoze apps that I have to use that don't have OS X versions!). I still consider my mid 2010 MacBook Pro to be a pretty new machine; to then find out that it is too old for this feature is VERY annoying. My guess is that the majority of OS X users will have hardware that is not supported for mirroring - it should have been made much clearer in the marketing that you need to have a VERY new Apple computer for this to work!
Yes, AirParrot does sort of work, but the slight delay makes it a bit useless for anything that requires quick screen refreshes, especially video.
After reading more about the reason my hardware isn't supported I can understand why, but it is bloody annoying that this was not made much clearer before bought Mountain Lion.