Were this edition another iteration of the 3rd-7th paradigm, my answers as always would be things like:
- Modifiers (of all stripes, to-hit, armour, etc.)
- Cover that didn't run in to things that should be weird edge-cases in every other firing phase
- Separate movement values rather than movement-by-unit-type
- Splitting fire
- The simplicity that comes from a flexible statline that can do a lot of heavy lifting instead of a long list of "universal" rules that only act as an additional layer of rules upon which more rules get piled because the basic system handles baseline mechanics poorly and needs to make a huge swathe of exceptions just to work properly before starting to use rules to make units more unique *coughwheezepant*
In a lot of ways, 8th is
40k bringing back the things that I have long lamented being gone while changing things that I have long railed against (I've wanted the
AV system to die since 2nd Edition and wanted it to die even more when
GW dropped all pretenses of the meat/metal split by letting bikes and certain walkers use the infantry damage system while keeping other things in the vehicle category despite that making them actively bad in comparison).
I might say templates, but I think 3rd handled the template streamlining pretty badly. The small blast was too small, and only a single flamer template meant that all flamer weapons were only differentiated by S and
AP. Getting rid of templates means we can have the flexibility of 2nd Edition's varied template types without having to carry around half a dozen blast templates (including the pinned-together thudd gun template). The visual was nice, but the compromise 3rd edition made for keeping the visual while gaining simplicity nerfed a lot of useful weapons in to oblivion.
In general, many mechanics in the 3rd-7th paradigm were handled poorly enough that I can't think of one thing that I'm sad to see go. Doesn't mean that 8th is an unequivocal improvement, but previous editions have set the bar very low.
Either way, 8th feels like the cleaned up 2nd I wanted back when 3rd was announced. I still remember reading the "what's changed" article in
WD and becoming deflated. The previews of 8th have had the opposite effect and for the first time in nearly 20 years I am interested in 8th edition on a mechanical level. So

the old paradigm.

the compromise mechanics born of 3rd Edition's penchant for throwing the baby out with the bathwater. And

the Frankenstein's monster rules remix that each edition that came after was alongside the cargo-cult design methodology without thinking about how things were supposed to fit together in actual play.
40k has needed a ground up redesign since its last ground up redesign and 8th is looking to be that spark it has needed. At least to this old grog.
Bring on the change I say.