What would make an app incompatibile with OS X Lion? Good ways to test or check the compatibility with Lion is being asked in a different question: What tools are people using to check for Lion compatible software?

What, generally, would make an app—that works with Snow Leopard—incompatibile with Lion? Could I have an educated guess whether app works / doesn't work / works partially with Lion based on some features?

What does, for example, all the apps reported "Not working" or "Has some problems" on RoaringApps Compatibility table have in common?
My educated guess would be:

1) Any apps that deal with operating system files

2) Any Power PC apps

These guesses are just based upon the information given in the app profile on roaring apps and about what apple has told me.