This thread prompted me to read the
5e Grease spell. Here's the core effect of the spell:
When the grease appears, each creature standing in its area must succeed on a Dexterity saving throw or fall prone. A creature that enters the area or ends its turn there must also succeed on a Dexterity saving throw or fall prone.
To me, this seems a microcosm of the issues with balance in contemporary, non-4e, D&D play.
The effect is of (roughly) equal utility at all levels - being prone is a debuff that costs movement to overcome, and there is no general tendency of higher-level NPCs/monsters to have immunity to the debuff (eg flying) nor to have more of the resource used to overcome it (ie movement rate). (The fact that the 5e movement penalty to stand from prone isn't as severe as in some other editions doesn't change the fact that it is a penalty that facilitates mobility-based tactics on the player side, as well as an immediate debuff against close combat attacks.)
But the cost - the expenditure of a 1st level spell slot - reduces dramatically with level, particularly given Arcane Recovery, and at the highest levels of play Spell Mastery. (This is the contrast with 4e - an encounter or daily power is an encounter or daily power, and there is no particular class build that reduces this to a negligible resource cost.)