Unfortunately it's not just games. It is:
- Terrible (if any) support for webcam / voice chat in messengers.
- Terrible (if any) support for 3G cards. I know the latest versions have improved 3G support, but most notebooks don't run the latest versions.
- A plethora of little but extremely annoying bugs which can be fixed with just a tiny fix in the text configuration files, that no non-adept will ever think of doing.
- Flash / java plugins for firefox that keep bringing the machine to its knees.
In general, I think that a) many features that have been considered a must in the users' world for ages are still unavailable in linux deskrop, and b) the rapid addition of new features to the latest linux distros (mainly ubuntu) have come at a price to quality and stability. I can't see linux gaining many more users from the masses until it addresses these issues.