I can't add anything new that hasn't already been said before. But like everyone before me I wanted to vent my frustration and join the growing chorus of angry voices in the hope that Apple may do something about this.
Of the various features of Mountain Lion, Airplay Mirroring was the one thing I was waiting on. It offered a really nice feature that was going to benefit my household well.
I appreciate that companies can't support legacy hardware for ever. However a Mid 2010 iMac is hardly legacy, particularly given it runs an i7 processor which is only now becoming standard. One of Apple's major selling points and what differentiate it from a PC is the longer lifecycle of Apple hardware. Apple's move to only allow some of the most desired features of its new OSX on the latest machines suggests they are trying to shorten the lifecycle and force people to replace hardware sooner.
So yes, as it turns out it is in the fine print tucked away way down at the bottom of the page. If that fine print was immediately below the section promoting Airplay Mirroring and easy to see prior to release, I would have been disappointed. But I would have probably dealt with it and moved on. Finding out by trying to use the one key feature I wanted, only to discover its not available to me. Like everyone else here I feel quite decevied.
I'm sure there are a lot of great features in Mountain Lion, however this has left a sour tast in my mouth.