Playing both again recently, I love the visuals of D2 summons and the high numbers you can get, but god damn is it annoying to have to manually summon everything over and over, not to mention summons that aren't revived are so weak in D2. I also loved how the visuals of the skeletons improve as you upgrade them.
I honestly liked how the mages were originally supposed to be in D3, automatically summoned like command skeletons, and with the ability to focus fire enemies.
D2
+Visual upgrade with skill points
+Minion master role play, lots of troops
-Summons, especially skeletal mages are very weak
D3
+Summons are auto summoned, this is a big deal for PvP
+They do more damage and have more health
+Ability to focus fire champs, enemies, and other heroes
-Weaker visuals, most of the rune types don't really change the look of the skill
-Skeletal mages feel cheap, more like an attack skill than an actual summon
I really hope we do get a hybrid of D2 and D3 summons for
Diablo 4.