Skill trees may be a good idea but don't forget that they will limit your skill points further. Think DiabloII where you had to spend skill points on low level skills to get higher ones. That means that you will have less points to spend on skills of your choice.
MCing in DD is too free, I think. It is so free that you can choose "Warrior" to start and end with a powerful "Mage". <img src="/ubbthreads/images/graemlins/biggrin.gif" alt="" /> There should be some disadvantages.
Egin, I can't see any downside of MCing. Only difference between classes is on secondary stats such as damage, defence/offence, life/mana etc.
I think that Larian should work on MCing some more before they release DD2.