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?