Your sentiments on your iMac is exactly what I've been talking about. Not the people who didn't read all the caveats before buying Mountain Lion. I didn't buy it because of its limitations.
I bought my 27" iMac on sale just THIRTEEN MONTHS AGO. It was still $1500 on sale as I got the most features that were available on the mid-2010 model. (why do they say mid? There's only one 2010 model...I just looked at a memory upgrade redirect to confirm that when clicking on the apple and "about this Mac" and "more info"). That's a lot of money to be obselete 13 months after I bought it! The 2011 model had just come out which was why it was on sale, but there was no mention of the fact that it wouldn't be able to have Air Play Mirroring in the future. I upgraded to Lion with no problem, and didn't forsee Mountain Lion being an issue in a computer that was purchased less than a year ago when Mountain Lion was introduced.
I can see a $500 laptop not being equipped to handle the upgrade and the investment is not so great in that situation. You can afford to buy completely new equipment more often if you're buying something cheap. But you expect when you're paying more for an Apple Product you'll get more longevity out of it and be able to purchase upgrades, if desired, with full functionality, as they become available.
The tech geeks on the site who probably make a living off new purchases based on new "chipsets", etc., they might've helped design obviously have no sympathy for us. They are like "stop whining and buy a new one".
But if Apple wants the middle class consumer to expand and grow their base, they are going to find that consumer is not the tech geek who "has to have the fastest, newest..." . I DO expect to be able to purchase upgrades 13 months after I spent a good chunk of change. Apple will fail in maintaining growth of that consumer base if they disillusion them so quickly before loyalty is developed.
It's a shame because I wouldn't mind spending $600-$800 to purchase the necessary hardware to have 2011 capabilities. But I will not be forced into buying a totally new computer after just 13 months!
If this is not addressed I will leave Apple entirely.