Personally, one of the attractive points about ADOM for me is that it strikes an excellent balance between fully classless systems (like Oblivion or Omega) and highly restrictive class systems (like Angband).
Classes offer many advantages: they provide unique abilities and challenges, improve the replayability of the game, make it easy to leap right in by relieving some of the management burden, and provide a useful hook for role-playing. (Yes, I role-play even in single-player games. I like to make decisions in ADOM from the point of view of the character, and another shining point of ADOM is that there are several meaningful decisions to be made.)
On the other hand, it adds a lot to the game to know that the classes are not totally restrictive. If my Fighter was born under the Salamander and really wants to learn magic, it is possible--but difficult. It is somewhat easier for a merchant, and even easier for a bard. This is as it should be. In principle, all skills and abilities in the game are open to all characters, with a few rare exceptions (Mindcrafters and Barbarians learning magic, advanced Necromancy).
I think some of the dissatisfaction being expressed in this thread stems from the lack of opportunity in ADOM. Herbalism can be learned from Guth'alak. If characters could learn herbalism just by grinding, they would have no strong motivation to go on the quest. Some skills can't be learned at all in the Drakalor chain except by wishing (and some not even then), but it's a small setting. If teachers exist for every skill in Jade, somewhere in the vast world, wouldn't that address people's concerns?
After years and years of role-playing, I've learned that truly classless games encourage characters to converge on the jack-of-all-trades. Everyone picks up a few spells, the best skills, and so on, just because they can. Already, in ADOM, nearly all characters become thieves because there is little reason not to--joining the guild is easy, and the skills are very useful. If there were social consequences, and certain quests were closed off entirely, the choice would be more interesting.
So all that rambled a bit, but here's my proposal: that Jade should use the ADOM class system. Each class has a certain raw talent for weapon skills and magic: low, moderate, or high. For some classes, individual weapon groups and spells can be tweaked for flavor (this already exists in ADOM). Each class has several key skills that they advance easily, while other skills are more difficult to advance.
It should be possible to acquire new skills, but not without cost. Finding a teacher means attaching yourself to an organization, which has consequences on NPC interactions, available quests, etc.
By no means should you be able to acquire new skills through grinding. If you can get an ability just by investing time, with no consequences, then there is no interesting choice--you must assume for balance reasons that every character will do this. Grinding is boring, and a game where you can become super at everything inevitably turns into a grind. Classes make the scope of a game more manageable.
My most recent character, Cugel the thief, worked long and hard at getting literacy up to 80%+. When I finally started learning spells from the books he'd been collecting (around level 20), it felt great--but I realized that I still was far from being a wizard. My power points regenerated pitifully slowly, and the spells disappeared from memory more quickly. Later on in the game, I was able to wish for concentration, which meant I could use my spells much more frequently.
In Jade, perhaps he would have had to find a willing teacher to learn Concentration, attaching himself briefly to the guild. Then later, since he was a thief, he might have betrayed them by stealing one of their artifacts, making a new enemy.
Cheers,
Ben