There's a pretty big difference between:
* Character uses up their best abilities, is now at 10% effectiveness
* Character uses up their best abilities, is now at 80% effectiveness
Also between:
* Character is holding off from using their best abilities, is at 30% effectiveness
* Character is holding off from using their best abilities, is at 80% effectiveness
As a caster, I find the 15-minute day a far more serious temptation in editions before 4e, particularly so if healing items (wands of cure light and lesser vigor in 3e, for example) are not readily available.
As a DM, I also find the power difference troubling because it makes it easy to cakewalk theoretically difficult fights or TPK on not intentionally difficult fights due to swinginess in player preparation and rest choices.
One experiment I'd have liked to try in 4E was to limit daily power use to one per fight - of course, I also wanted daily powers to always be a big encounter changing deal (no Brute Strike or Fireball, yes Evard's or Consecrated Ground) so that may have been a non-starter, but I'd not mind a similar concept of throttling how fast a 5E caster can expend their load. It does make sense that there is some limitation to throwing everything you got all at once, after all.
I wouldn't mind caster spell slots being balanced around getting them back steadily throughout the day - then you could still let things be wacky or strong, but not terribly strong. I also wouldn't mind them having a variety of at-will spells and rituals and _very few_ big spells they can use per day, more true to Vance and early level D&D. (Nobody really needs 60 spells memorized, do they?)
Anyhow... back on topic a little:
In pre-4e, I found myself largely playing casters. In 4e, I played anything. I think because I need a certain level of complexity and tactical buttons to push to stay interested.
So, even if it's pushed into an optional rules module (which is fine), D&D Next has to deliver some of the cool things I've been doing in 4e with non-casters. It doesn't need Come and Get It - that was frankly a mistake for fighters to get (paladins? sure), and it doesn't need marking, or quarrying, or cursing. But I would like the ability to decide my character is a swashbuckler who is about tumbling about the battlefield and knocking enemies about and over... and not be horribly ineffective, consigned to non-damaging and ineffective bull rushes and trips, as a result.