Firstly, you no longer receive stat points and skill points as in the original Warlords Battlecry 3, but instead receive ability points (APs) like in Warlords Battlecry 1, which can be invested in several/all of the heroes' ability trees.
There are now four ability trees. The first two remain the same, stats and skills, and TPC comes with the new Perks and Spells tabs.
The Perks tab mainly represents the personality of your hero. It reflects its psychology and how it prefers to play. The list of attitudes is determined by race, class and proficiency. Basically, it has 6 pairs of extremes which are in effect whenever the hero is alive. At any time the hero can change his/her way of thinking, provided he/she has sufficient APs, thus changing their preferred gameplay style. For example, the Fey hero can be either a Pessimist or an Optimist. If he/she is a pessimist, then the army always suffers a morale penalty but has high resistance to enemy critical hits. He/she can always choose to be an optimist and then change back to pessimist, but the cost of having such an unstable personality is great (loss of APs and levels). Therefore, it is usually good to stick to a certain setup right from the beginning.
The Spells tab was inspired from wbc1. Like the name says, it is used to level up your hero's spells. The twist is you can now allocate different points into different spells and spell spheres without going in the restricted straight line of the original game. Following a branched system, it is now fairly simple to have only the spells you like of high level, rather than investing in all spells of that sphere to reach the one you like. The system is not totally liberal, i.e. you cannot have Resurrection right off the bat, but it does offer more maneuverability without loosing game balance.
With this many places of investing your precious APs, leveling up your hero will be that much funnier. Deciding what is the most urgent need of your hero will now not be so easy, nor monotone.