Quoting Peace Phoenix, reply 7
It isn't the same to be able to teleport in your territory (defensive move) vs teleport into ennemy territory (like you did in HOMM3)
It's not quite as powerful, but it can still be used offensively. You can teleport your army to a border hex and attack anything near the border. At least in Civ, you could take over cities this way.
And besides, the true fear here is whether there will be a full teleport spell in the game...
It's not just teleporting - many spells will allow an army to get where you didn't expect it to be, if used creatively. Lowering a mountain or raising land out of water, for example, can eliminate natural barriers and allow an army to unexpectedly bypass defenders stationed at a choke point. Not to mention spells that can change the odds of a battle, letting a weaker attacker unexpectedly win against a defending force that should've been sufficient (almost every spell falls into this category; summons, direct damage, etc).
In other words, nearly any spell can throw what you expect to happen out the window, by allowing attackers to beat superior defenders, or bypass them completely. So players and AIs essentially need to predict what spells the enemy can cast and account for them - you may not know if your enemy has lower mountain, so you'd better count on him having it and expect armies to bypass mountains. While you're at it assume he can teleport, rain ice on you, and bolster his numbers at the last moment with a summon or three, and act accordingly. "Expect the unexpected" indeed.
It almost makes you wish there was a way to determine what spells an enemy knows, or at least what spell books and/or shards he has access to. Perhaps with a spell..