The skill tree is what truly diversifies the classes. Each class has a unique skill tree with several branching paths. You can get a new skill every time you gain a level, but you'll never be able to get every skill in the tree. Again, you'll have to choose which will best fit your play style. Take the medic, for instance--one path makes him a more efficient fighter, while another increases his ability to treat his allies' wounds. If you prefer to take your time and carefully complete each mission, you may be able to complete each one without sustaining any serious injuries to your troops, so that you don't need an exceptionally good medic. You might, instead, opt to improve your medic's fighting ability to deal more damaging critical hits--a random occurrence in battle that deals exceptional damage to enemies. In any case, when a soldier's vitality points are reduced to 0, he or she merely becomes unconscious. You can't revive characters during the mission, but they'll be alive and kicking in the next one.