Save-or-die / save-or-suck effects are a much trickier subject. We all rely on the monster manuals or published adventures occasionally when we're low on prep-time, and if the published material is full of monsters who stun/paralyze/banish/kill pcs with a single attack, that makes it a major head-ache for me, as I find that type of gameplay very frustrating and un-fun. Yeah, I can work around it and not use those monsters if I have to, but if that mechanic is built into the core of the game, it makes the game a lot less fun for me and makes me question why I would give up 4e for the new game.
I understand that a large collection of players (like the OP) feel the exact opposite way and would feel like things were less fun without paralyzing Medusas, or the cleric being able to banish the pit fiend in one attack if it fails its save (pick your example of choice). That's why I said in my first response that it will be impossible to have an edition for everyone. If WotC can manage to come up with a system that can make players on both sides of the save-or-suck divide happy, it would be amazing, but I'm having trouble imagining how you would do that.
Thresholds. If you have a cleric like in your example who focuses on turning or banishing, etc. at a certain point he goes from doing x damage to banishing or destroying the thing outright.
The downside being you don't want the game to just devolve into everyone trying to hit some kind of required threshold. So it should be mostly unattainable except to extremely focused characters. Stuff that should be completely unavailable to multi-classed characters. I think the capstone abilities in pathfinder touch on this idea but I think it could be extended to include abilities that are strictly made for characters who only stay in one class.
The way 4E is setup as far as classes sucks imho. It's not interesting. The multi-classing feats were a joke. Hybrids work but only sort of. The required stats are too extreme in 4E. A system with multi-classing but that gives specific benefits to staying in one class is a great idea and I think one way to integrate a lot of the old style mechanics.