My suggestions:
1. Beef up the effect of Stealth skill for skill levels above 75.
2. Let choice of armour affect ability to be Stealthy, based primarily on metal type. Non-metal armour should have no effect on Stealth, lower metals should have more effect (greater reduction of Stealth) than higher metals, and there should be an ego type that nullifies noise and thus allows metal armour to be like non-metal armour with regards to stealth.
3. Let light sources cancel out stealth.
4. Make Stealth even more effective for thieves and assassins (and maybe rangers), while perhaps making it less effective for merchants, bards, farmers, and priests. Why? Thematic effect - merchants, bards, and priests are characters that should want to be noticed, while farmers would have no experience with being stealthy, since they usually work with domesticated animals and with plants.
Note that I don't want merchants, bards, and farmers to be even harder. I would want to see counterpoint modifications to the game to give them a few additional benefits.