Many many of the things already listed are high on my list but a few I've not see are:
- The removal of body slots. Attunement was a neat idea but it would be kind to say that attunement use/no is random at best leaving me as the GM to finish that and keep reminding players "no you can't use the +N weapon/armor or whatever from the DMG because yours is attune required". Body slots allowed me as a GM to provide more depth to magic items
- Kind of a corollary but the removal of bonus types means that I can't limit the power of an item by using a bonus type easily obtained elsewhere like the +1/+2/+3 armor/shield/doodad of protection that all stack with only one actually needing attunement even though most if not all of them in the past were an enhancement bonus that limited their impact.
- The removal of Flat DR/flat resist & SR. These three collectively allowed monsters to survive more than an eyeblink against a big group. I could add flat DR & maybe even flat resist but I would need to rework every spell & ability to decide which were SR:Yes & which SR:No to add SR back. bolting on SR is not something that I can do without basically rewriting the entire game.
- The removal of (Sp) (Ex) & (Su) tags on abilities that covered how they interacted with things like antimagic & were subject to AoOs (or not). Someone at WotC removed them in the name of simplicity but it didn't take Wotc long to give players racial/class options that did things like grant advantasge on saves against magic effects & similar that would be covered by them.
- the dual competing long rest class/short rest class desires coupled with an unreasonable 6-8 encounter expectation thrown at the gm to simple solve is rage educing.
- I hate the virtual removal of tactical grid combat & having variant/optional DMG rules for it that seem to be written more to thwart making tactical grid combat a meaningful factor as anything but free buffs