I have one site that I am not upgrading from 2.6.5 because (a) it doesn't need threaded comments, (b) Ozh comment plugin was better than the core replacement, and (c)the 2.7/2.71 UI is slower and less usable.

With every releases since 2.6 I've found myself using more custom functions and plugins to override the core.

With my 2.7+ sites I am blocking auto updates, filtering out the generator tag, wpautop and wptexturize, turning off wlwmanifest_link and rsd and blocking all dashboard feeds. Post revisions are customised. I neither need or want widget management from the core so will probably disable this in 2.8 as well.

When it comes to what the software does for my sites I like to have complete control ;)