Also, no up-casting, down-casting, or metamagic. If you're out of 4th-level slots then you can't cast any more 4th level spells today period stop end.
Corollary to this: spell effects/ranges/etc. go back to scaling by level, such that a Fireball cast by a 10th-level wizard has more bang to it than one cast by a 6th-level.
As for other classes: I'm lumping all the arcane types under "wizard" here, so it applies to Sorcerers, Warlocks, etc. as well. For divine types, the interruption piece should be there; but as they're otherwise nothing but a conduit for powers granted them by their deities, I'm fine with their having the full list to choose from.
Bards are, of course, an outlier; and IMO need to be reseigned from the ground up using bespoke-to-the-class mechanics involving sound and sonic magic that no other class has access to.
Oh, and while we're at it: multi-class casters of any type should lose some (half?) of their spell slots, to discourage multiclassing.