Native applications will not be making it to the Lumia 900 and as such your going to start seeing more and more apps (and especially games), not be able to run on the phone.
This is different in that it's nothing to do with hardware. A super simple contrived example would be if someone builds a calculator using c++, you can't install it even though there is no hardware reason that you couldn't
Look, no matter what phone or OS is, there will be something that you won't be able to run unless you want to update every piece of hardware/software every few months. There's always a trade-off if you are not trying to be "current" at all costs. It's OK, there're more important things in life than an ability to run a piece of software on the phone.