OS X has changed very little since 10.0, at the most basic level.
Yeah... no. They broke cron, they inflicted that insane "app nap" nonsense on us (broke damned near every real-time application out there... I spend a *lot* of time explaining to OS X users that it needs to be turned off or OS X will summarily stop giving the required amount of CPU time to the app) there's sand-boxing, the changes in spaces functionality, they utterly broke UTF-8 console printing (and didn't fix it... just left it broken unless you upgraded -- and yes, they knew about it in time, I talked to "Mr. CUPS himself about it), dropped PPC emulation, moved image support from apps to OS (which broke the dickens out of Aperture upgrades, among other things), they broke getting to local websites on your LAN, and they quit giving us actual media, which I simply find annoying and short-sighted. And they still haven't fixed many of the OS bugs, for instance, you still can't have more than one app listening to a UDP broadcast reception port as far as I know. I don't have any idea whose brilliant think it was to decide that "broadcast" meant only one app can listen, but there you go.
Definitely quite a few reasons to be reticent about moving to a new version of OSX. These things matter.
Anyone familiar with OS X 10.5 would be right at home with 10.10 Yosemite.
Sure -- if you don't mind a good deal of your stuff breaking. Inconveniently enough, I do mind. Hence, 10.6.8, and staying there as long as possible, too.