This is my first comment on the support site. I have to say, I am equally disappointed with the lack of compatibility and lack of truthful advertisement of the new features that came with Mountain Lion.
I am a devote Apple fan and generally trust them blindly with updates. My iMac, which I bought in 2010, was compatible with the new OS upgrade so I purchased it in the App store without looking too much deeper. After I nought it, I started trying out the new features. Unfortunately, I am in the "my iMac is too old" group which I find to be tremendously disappointing. My iMac runs great and I feel duped for buying it, now knowing that Apple does not feel my purchase of less than 2 years ago is worth the extra effort to add compatibility with new software too.
Unfortunately for me, my iMac was a huge investment, which I saved tirelessly for, so I will not be able to purchase a new Mac of any kind until this one really doesn't work.
Overall, I am feeling pretty sad, as I give a huge percentage of my technology purchases to Apple, and right now I feel like they don't give a **** about me.
I have an iPhone, Apple TV, iPad, iMac and Macbook Pro. So far just my new iPad and Apple TV are compatible.