I view casting 3rd level spells as a significant mechanic more so than how they cast it.Ah! Okay I figured it out. You view the spell slot as a significant mechanic.
A bard, wizard, sorcerer, and warlock walks into a goblin cave..
one casts hypnotic pattern
one casts fly
One casts major image
One casts slow
And you can't tell who did what, because they all cast the same spells.
Divine spells have 1.5 classes that can cast them
Nature spells have 1.5 classes that can cast them
Arcane spells have 4.5 classes that can cast them.
Multi-attack has 5 classes that have (while I'm counting).
I would rather warlocks be their own thing than just another arcane caster variant.
A lot of this had to do with classes losing their unique spell lists.