I'm going with 'hard' classes in neohack. Every class has some ability that other classes don't.
For the first version I'm trying to implement fighters, magicians, and thieves.
I will be using very few class/item restrictions as such, but each class has important, survival-level abilities that they will be unable to use if they kit out with items that mess those abilities up.
Magicians won't want gear that's made of iron or steel, because those metals are anti-magic; prolonged contact with any iron or steel will drain their mana to zero. This means that effectively all the 'heavy' armor and weapons in the game are unusable to any character that wants to use spells. If you're a wizard, you'll kit out with an obsidian or silver dagger, a staff or bow, and probably leather or padded armor. Or you can give up spellcasting, which leaves you with few advantages.
Likewise, thieves have stealth as a special ability (everybody can have some stealth, but thieves have lots of it if they try) and devastating attacks from stealth. This makes them much more survivable as characters; so they want to avoid noisy gear, which category, at least in my ruling, includes most heavy armors because those armors impose caps on their stealth. So, like the wizard, they either give up something central to their survival, or they avoid the gear that restricts that activity. The noise constraint does not rule out nearly as much of the best gear as the 'iron' thing, but it's still significant.
And finally, there are fighters. Yes, they use all that heavy gear. They have little stealth and no spell-casting ability in the first place, so it's a non-issue. The fighter is awesome in combat because he gets a special ability to move and hit on the same turn - meaning he can't be pillardanced, can't be maneuvered into giving enemies a free attack due to turn-taking because he has to move to them, and has the options of taking free attacks while retreating and free attacks while maneuvering. At high levels, he even gets a "knights move" option which is a jump exactly like a chess knight makes, making him damn near impossible to surround or cut off from escape. So fighters, if they don't get too hurt, can dominate meelee scenarios like no other class. And what do they need to not get too hurt? Heavy armor, that's what. Iron or steel is great because it even gives them some extra protection against magic. For exactly the same reason the magicians don't want to use it.
