Like what? Vision cones? Alert meters?
Off the bat, a stealth game has to actually be completable by stealth.
And yes, functioning and defined vision/audio mechanics/physics go a long way. You need a ton of level design, enemy placement, and logical AI to make stealth games work as well.
Since we're in the Sony thread, lets take the LAst of Us.
- Full on forced open fights with the bloaters and at various other points.
- Omni-AI,. one alerts and they all have psychic bonds connecting them.
- Basically totally under-designed levels. Last of Us is corridors and waist high cover ad nauseum. No alternate routes or anything that would really enable sneaking,
- IF there any kind of system for light-detection, its awful, also for any kind of camoflauge.
You need more then letting your action-game character hide behind a rail and instakill one or two guys to implement stealth mechanics properly.
The most laughable case there being Skyrim. Skyrim doesn't have stealth mechanics at all, it has a passive magical ability to turn invisible by crouching, with the effectiveness bound to effectively a dice roll.