I gather a lot of people are struggling to understand what does and does not transfer over when you change classes, so here is a summary:
1. Weapon skills trained by one class are available to another class IF the new class can use that weapon. Note that this rule is overridden by the 100 point ability in the weapon skill trees, the "Omnivocational" passive. So if you really, really want your Gladiator to use a wand you can make that happen, you crazy fool. Note this also applies to shields, meaning you can make a shield-bearing Martial Artist...
2. ALL VOCATIONAL SKILLS TRANSFER OVER. I'm guessing this point is being lost on people since no one is discussing the benefits of, for instance, stacking the passive bonuses to Magical Mending from priests and Magical Might from mages in anticipation of building a Sage. Once you learn a vocational skill or passive bonus, it's yours forever.
3. VOCATIONAL SPELLS DO NOT TRANSFER OVER. This point is obvious, I think, but it's the confusion between spells and skills that makes point 2 above a little hard to grasp. Spells are the abilities granted to you automatically as you level a class. Skills are the abilities and passives granted to you for spending skill points. Your mage will lose "Fizz" and "Crack" when he changes to a Thief. He will NOT lose "Wizard Warding".
4. When you transfer back to a class you previously abandoned, you will regain your EXP, your previous level, and all your previous spells. This encourages dabbling in other classes in order to strengthen your intended main class... For instance, flipping a Priest to a Minstrel in order to build up the Shields skill, then flipping back with all you old stuff and better defenses.
5. And, of course, unspent skill points are never lost in transition from one class to another, meaning you can "bank" them and grind on unfavorable classes to harvest skill points for use in another class.