I think it's been said before, but the Teleport spell is far too powerful for its cost and can make the game too easy for a Kingdom faction. There are a lot of complaints about the AI, and I have mine too, but in a recent game, the AI actually put up a pretty good fight (on Extreme - I had to leave him alone for a while to get strong too) and approached two of my cities on his border, which were relatively far apart, with powerful stacks. My sovereign power stack was 5 moves away via normal movement. Without teleport, my two cities would have been taken, but with Teleport, I was able to defend both in one turn, using so little mana to teleport  that I had plenty left for combat.

Teleport should be a very expensive emergency type of spell, that costs like 25 mana or costs essence and requires advanced research. It can really unbalance the game and allows the Kingdom faction to operate without a lot of strategic thought to their defence. Teleport allows me to station small garrisons to defend against roaming creatures while maintaining one power stack that moves around crushing things. Without teleport, I'd have to station more troops on the border and create regional armies or risk losing cities, but as it is now, that depth of strategic thinking isn't required.