What I'd like, along with a base of some more functional stealth (no "psychics" or any of that), is for assassinations to NOT be scripted. Sneaking in through the basemant, finding a passage in the wall, and untying a fixture at a certain time of day; that's great. Or it would have been, if *I* had figured it out, instead of being told to do exactly that step by step. Or otherwise being told with astonishing accuracy where my target is, what their schedule is, when they are vulnerable, how to provoke them, and what I need to do to keep it secret. It's boring. I want to be given a target and then stalk them myself, keeping watch from a distance, observing their daily schedule, asking around town about them for any useful tidbits about their personality. I want to climb a tree and then kill them with an arrow because I thought of it as a way to bypass their security, instead of just mindlessly following directions like any other chafing fetchquest.
Instakills with stealth would be overpowered.
Depends on how it's implemented. A number of people (like myself) would like health in general to be lowered from Oblivion, so that fights aren't who gets hit 500 times and runs out of health potions first. A good stealth kill would be someone caught by surprise, possibly asleep, unarmored, by a skilled strike. It wouldn't be something you could just casually apply to everyone, especially if things like harder security access and getting away with crimes were implemented.