As mentioned in my post on the discussion forum, I think Z'dar is way overpowered and must be picked for the best chance of victory. He should be nerfed a little so the other heroes have a chance to shine.
Melee heroes need a massive buff, having to walk over to creeps to hit them is a big disadvantage and should be balanced out with much better reach, DPS and armour.
Finally, Thea's Astra Force does around 150 pts of damage which is puny compared to Z'dar's Earthquake which does 10 times more damage on a good day. Earthquake also stuns for twice as long and hits the entire screen which Astra Force barely covers a quarter of the field. Both of them are the max upgrades for the third active skill but why the huge difference in power?