In my missions' descriptions I tried to write, what abilities were learned by my heroes, but now I know that I would spend some points in a different way. I won't describe now all skill sets, but I will only mention about most important ones and ways of development.
First of all - there is no active pause in this game, so it is hard to use too many skills that need activation, especially when they need a target. So I decided to invest in passive abilities as often as I could. Besides, they don't use our magical power.
Secondly - five spells that you must have (if it's possible): Fireball, Father of thorns, The undying servant, Favoured minions and Steal soul.
Thirdly - it's good to invest in passive skills from command tree (but such as Pathfinding or increasing gained experience are rather useless in my opinion - buy them only, when you need them for better skills). By looking at skills hero has you can decide which regiment add a hero to.
Fourthly - most of the time you don't need to spend points on skills from Duel tree. Our heroes will probably have higher level or better equipment then their opponents and we can harm them enemy with our troops first. Thorgar or Stefan can have some points in those skills, because they will have many occasions to duel.
Fifthly - order. In most cases mages should start from combat and fighters from command tree.
Sixthly - if you have problems with transferring items between heroes, just apply newest patch - it repairs that bug.