Woah now, those are fighting words!
I am going to argue for the approach used by 1e. Differentiating the spell lists by caster, and having each caster have completely different spell lists ... it really made the spell lists different.
It also (in 1e) kept the spells from overlapping that much. Sure, there was some overlap- druids and clerics both had (for example) Cure Light Wounds. But I would say that the spell lists kept the four classes distinct- spells rarely were exactly identical, and often would be different depending on the caster (famously, reincarnation).
I also don't think that the differences were arbitrary. There was usually some type of reason for the differences, although I admit that you might need a post-graduate degree in Gygaxology to discern the reason behind the lengthier casting time for cure light wounds for clerics as opposed to druids.