TB - Spells tend to top out and then he is really only useful for creep bashing (but oh how fun!). Deep Freeze's shatter might have got nerfed a little too hard. Granted, - cooldown items really help, but until favor is fixed it hurts. That favor item makes a big difference in dps potential.
QoT - She does well what she's supposed to. I would maybe increase the cooldown, but add more damage for her line of spikes so she has at least some anti-dg protection. Shield may need to scale more. Same with Sedna's heal.
Sedna - balance is good, she may be a little bit too durable...I might add an AoE to 100 a rank earlier for her heal. Rounds her out a bit better
Rook - I think is well done, I might tweak his starting armor back up a bit and one of the cooldown/casting times on either boulder roll/slam might need a bit of reduction. Makes his dmg potential a little more reliable.
Erebus - Everyone hates bite...not sure the solution. I think if you nerf it too hard, he'll be very hard to play effectively. He's kinda the ninja general and dark's answer to sedna. I think light players need to discover how good she is and people will stop complaining about Erebus. His bite is really his only answer. I see him as the dark's answer to Oak. He can stay up in the front and melee, but he's fragile. Run out of mana, mistime bite...you're toast. Bring pots and simply outlast him. Nerf him to hard and then the dark doesn't have a decent melee general.
UB - I don't think spit should effect buildings. Also spit may need a bit of reduction in total dmg...or what if the DoT got bigger the more effects you stack? So reduce normal spit, but make it do more if target is oozed/grasped? Would make sense with Ooze and Grasp, and reduces the harass/easy kill factor? Kinda like TB's shatter.
Reg - balanced. At least for me. I think he excels if you play to his strengths and keep your build/equip focused.
Oak - Fine...I think perhaps soul power should only trigger extra dmg if the spirits are within a certain distace. The whole send the spirits to the back is a bit cheap imo.
Items - Heart may need some adjustment. I'd also reconsider the way crits stack and some of the other benefits. But maybe not. It's a viable way to play. I'm in the air on items. Again, they can be countered by adjusting your build. Again, I think some items that add a % increase to skill dmg are in order.
Sorry for the double post...all these thoughts suddenly popped in there.