The one unit I think was a classic of example of "if it ain't broke don't fix it" was the Scout. I like adding elements to the game, but I feel like the Archer unit should basically emulate that. It was a high skill unit that wasn't too OP and could be used well or poorly.

The one unit I think got really porked was the Stone Golem. It was an awesome idea early on, and the centerpiece of the turtle formation. But the Mudquake decimated it, and it became a useless unit (which is one of the principle problems with how TAO was developed). I would LOVE to see units that were pure utility, but could remain effective without a GA throwing a hail mary over the entire map to render it useless.

Edit: Point being, I feel like the main philosophical approach should be balance.