What warlock? Which edition besides 4E has featured a core warlock?
It's both the thing I miss the most and the smartest major design change made.I miss having wizards and other spellcasters that are able to do more than just blow things up in combat. D&D, for me, included wizards being magical swiss-army-knives, with multiple cool spells usable for many different purposes.
Don't you mean 'creative problem creating'?... but I miss being able to use magic for creative problem solving.
All previous editions - the ability to make a simple fight that took only 10 to 15 minutes to resolve and "get the blood flowing" without taking up a quarter of the game night.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.