8th fantasy was actually a massive improvement to the WFB hobby.
How so?
Well probably the three biggest changes imo were 1) changing how combat phase worked, 2) balancing the magic phase, and 3) making good ol' infantry not suck anymore
1) In previous editions only the 1st rank could fight and if the 1st rank died before attacking then the rest of the unit couldn't attack back. In 8th they added step up and supporting attacks and now combat works "similarly" to combat in 40k.
2) rebalanced magic phase so the more "magical armies" like DE, Daemons, VC couldn't spam magic against the magic-lite armies
3) Previously cavalry dominated most army list (the same way mech dominates in 40k) and infantry lists were generally pretty terrible. In 8th they gave infantry a major boost as mentioned above as well as the steadfast rule, which made it more difficult for elite units to break lighter ones.
Anyways that's only the 3 biggest but there were a ton of other changes too which made the game more balanced too.
As far as 40k is concerned, in 4th edition transports were death traps and tanks were way easier to kill. 5th is still better then 4th was imo, but I do expect mech to be far less powerful then it is now, just like WFB cavalry. I'm very excited for 6th, I expect lots of cool changes and I'm sure lots of lists will become more dynamic too.