Why though? What's intrinsically imbalanced about them?
Warrior/Combat
Warrior is, in my humble opinion, the most lacking of the 3:
1) Lack of skills that deal with obstacles like locks, etc. That means using a magic item (not necessarily bad) or leveling a minor skill (bad times).
2) Weapon wise, a player will typically use only one type. Bethesda has made the same mistake in skills (One Handed and Two Handed bloat the Warrior skillset)
Overall, the warrior skillset has been gimp, inflexible and repetitive in 2 games thus far. Will the trend persist in Skyrim?
Mage/Magic
The Mage is quite a contrast to the Warrior:
1) Tons of versatility. Can sneak around as well as a Thief, and endure as many hits as the Warrior with defensive spells.
Can do virtually everything that the other archtypes can do and more. There will be incentive enough to play a pure mage in Skyrim, perhaps too much.
Thief/Stealth
The Thief, overall, is actually closer to balance in my opinion. Moving Alchemy to Stealth was a good step in balancing the sets, but here's some possible future issues:
1) Speechcraft and Mercantile bloat the skill set. It makes sense to combine them
2) This is speculation, but Thief melee potential, either in sneaking or in the open, has to be observed. A thief shouldn't hit as hard as a Warrior, but a Thief should have a CHANCE of winning if exposed, even if it is slim. Stealth may not always be an option, and who would use a dagger against a Dragon?!
From what has been shown in interviews, dagger kills are really leaning on sneak attacks (10x damage modifier!), and this could be problematic.
Conclusion
As I've said, Classes may not exist in Skyrim, but that doesn't change the fact that people will still decide to focus on pure builds. The skills from each Specialization need to ensure that each archtype can stand on it's own while still retaining weaknesses and unique playstyles.
Thoughts?