I'm a Safari user (better battery usage for the # of tabs I have open) and it too has improved with El Capitan though that's irrelevant for Chrome/FF users.
Worth mentioning that airplay is just userland software - nothing special, and no reason it couldn't have been added to SL.
I don't know about multi display, though - I've been under the impression that that is broken in new and fascinating ways with every single release...
Then Apple broke it massively in Lion, and only finally resolved most of the (severe, productivity-destroying) issues with Mavericks.
Also SL mamed Expose (that weird non-proportional grid view) that was reverted to the Leopard-style in Mission Control (of which Mavericks/Yosemite had the best implementation, and they've now broken its utility in ElCap thanks to hiding thumbnails by default. FFS.)
But apart from that... I think I preferred the Apple apps back in 2009-or-so.
To be honest, I think the latest Apple release cycles have been more about "remove a feature so that we can add it in again and sell it to our users again". Think multi-monitor support, something that worked perfectly in SL and earlier, and then broke fantastically with the full screen apps in .. Lion? ML? One of the two.
Right - which is why we have all of the snow leopard nostalgia: because none of the newer releases have given us anything substantive that we really needed to justify the hassle and the bugs.
I am trying to think of something - anything - that compels me to upgrade SL on my mac pro, and all I can think of is that nifty take-a-picture-of-your-signature in the Preview app that you can then insert into PDF documents.
Ok, and maybe USB3 ?
That's all I can think of.