Check out the listed percentage to hit and be hit for your offense / defense (this is also dependent on your level). Personally I would get my agility up sooner if I started a new game, even with a melee character. If you start hearing 'swish, swish, whack, swish, swish...' when you attack, it is time to increase your agility. <img src="/ubbthreads/images/graemlins/winkwink.gif" alt="" />
differences between classes (secondary stats)
Monster resistances tend to get higher towards the end of the game, but Poison Weapon should be useful for some time, though I never tried it. I also did not max Know Creature, so am not familiar with which monsters are particularly weak or strong against poison. I found Stun to be handy, though having any two of Frost on a weapon, Stun on a weapon or the passive Stun skill can cause problems (enemies can hang mid air, not completing their death animation). DAD found that in this case an area effect spell that does not need a target, like Hell Spikes, can finish them off.
I found Spell Shield and Aura of Guarding to be quite useful in the middle of the game, though I bought spell books rather than put skill points into them. Kistandalius or Blake in Ars Magicana and Corinna or Mpenzak in Verdistis sell spell books.
I maxed the resistance boosting skill, though might not have done so if I was short on skill points. Warriors tend to have lots of spare skill points, so you do not have to worry about picking the best possible choice with each one.