I agree with what's been said about the trees needing some rearranging. Here is my input on the trees.
Warrior - The tree is boring, and even though there are some flavorful or unique abilities on the tree, you just know that taking lethal 1,2,3,4,5 is just more efficient. I would definitely break up the lethal line from being just a straight 1->5 and change it so you want/have to take some other skills. The specific weapon skills chains need some love, partly because they are competing with the lethal line. Add in a reduced cooldown on the bash and cleave to the mace and axe skills, and give swordsman something like an extra counter or a weapon ability for a master swordsman would be cool.
Mage - This is my favorite tree, but I think the evoker section suffers from the same thing as the lethal. At least you don't feel compelled to take it because you can focus on summoning or utility spells. I also wonder if the role of having a spellbook is diminished some by the trait tree. Also I think the Warlock profession should be changed because the trait tree can already add enough damage at will and could use more flavor/balancing.
Defender - My 2nd least favorite tree that needs a lot of work. Spell resistance chain is too long/linear. I suggest taking stun off the end, and making obsidian guard an offshoot chain. I would also add in some traits that actually improve this unit's defense aside from defending. Warrior gets traits to improve base damage so a defender should get traits to improve base defense. The tree ending in shieldwall is long also. One thing I do like about the tree is that there are some nifty abilities in there, like guard, withstand and rescue. These are the kind of abilities that make combat fun. Also revenge (+1 damage when hit) would fit well in this tree somewhere.
Assassin - I think this tree is better designed than most, by looking at the layout, but I also have not used this one in game much yet. I would suggest more with using a ranged weapon.
Commander - My least favorite and I think you need to rework the whole tree and concept, but I think that is already being done.
General - I am not a fan of potential line.
Abilities that you can use in battle are the best kind of trait, chains that are too long are not good, and that you should never feel forced into taking something. You could also add level requirements to some of the abilities in the chains, like you did for mages, if abilities are so strong that you want to delay a player taking them but you also don't want to make the tree too long, just to force a player into taking crap to get there.