1) They nerfed the builds that stacked the -5/+10 feats with bonus action, extra attacks etc which is good, since if you werent using those your martial was crap. On top of that they made casters better, so caster/martial gap is a bit wider.
2) Problem spells: at 3rd/4th level, casters just get win buttons. Polymorph giving 180 (temp) hp is a problem. If your 1st turn banishment lands, it can just end the encounter.
3) Personal one, but I played a druid 1-20. I used summons but never more than 2 and had my dice ready, so my turns weren't slow at all. Conjure animals made bears, eagles when the party needed to fly, giant octopus when we were in trouble in the water. Conjure woodland beings made dryads who could heal, stabby tiny guys with a move of 60 etc.
Both of those spells are now just spirit guardians.
4) The new meta of Emenations: for a martial, esp a monk (who can grapple without losing speed), the most damage they can do will sometimes be to pick up a caster who has an emanation going and drag them around. Its silly and dumb. A 1x per round limit instead of per turn would have solved that.