It's all just OS 10
Apple just expect you to run the latest version of of MacOS10, you can run it on quite old machines. The hardware requirements for 10.10 are no different to 10.7
yes it's daft that Apple sell machines that you can't upgrade the RAM on however I can't think of a single one of those that wouldn't have been supplied with enough to run the current version.
If you're running an old computer that's 7 or more years old then you're not really likely to be that arsed about running the latest and greatest software so unlikely to rush out to install the latest revision of the operating system.
Personally I've found that 10.10 hasn't been quite as polished as earlier revisions but the current patch does seem to have brought it back to its old self, of the machines I've tried it on I've found no issues yet.