The recent rounds of speculation about OS 10.11 suggest it’ll be a catchup release, in the spirit of OS 10.6 Snow Leopard back in 2009. Instead of packing in loads of new features, some seemingly barely tested, Apple may focus mainly on stability, performance, and some improved under-the-hood tools for developers. OS 10.6 was considered to be the true reference release of OS X and a true successor is overdue.
Snow Leopard was also the last version of OS X that let you run PowerPC apps on Intel-based Macs courtesy of Rosetta. Lots of support for older Macs was removed. It was restricted to 64-bit Intel Macs, meaning the earliest supported Macs appeared in 2008. Quite restrictive indeed, but it made sense that Apple wanted to clean out past bloat, and move on.
Now there are clear reasons why some Mac users stuck with Snow Leopard. One is the fact that they were not interested in ditching those PowerPC apps. Another is that they just plain did not appreciate that Apple added some iOS-related interface niceties to OS X, although the changes were mostly minor.