Imagine being able to instantly and cheaply move a tiger tank platoon anywhere in the world. Or the Yamato and 4 carriers + 7 other ships anywhere in the Pacific (or Atlantic or North Sea or...). That's the analogy that describes how the cheap teleport is over-powered.
Ok, the analogy is close but not quite right since the teleport does in fact cost 15 now. A sovereign isn't going to get much beyond 15 mana early game unless he doesn't imbue, so the analogy is: "teleporting a bunch of tiger tanks minus their 88mm ammunition".
From another angle: it takes 15 turns to regnerate mana, you should easily be able to get your sovereign moving 4 or 5 with buffs (boots of travelling AT 10 bucks a pop is one easy way). 15 mana = 15 turns regenerating = 120 squares of movement on the board on caravan roads at 4mp or 150 with a 5mp soverign (this is how far I can move while simultaneously retaining my mana while you have to "regenerate" lost mana).
I play on large maps, ridiculous setting, I probably only average 4 or 5 teleports the entire game since they've nerfed mana at 15/pop. It's just not a game breaking spell to me. Like I said, I agree that it's a powerful spell, but it's definitely not "over" powerd. If the AI were programmed competantly, it would take advantage of an army that has just teleported to the border & crossed by moving a stronger army with archers to take advantage of the now "mana-low" attack force & target the spellcasters with archers. A second follow-up attack (assuming the first archer attack failed) that can now take complete advantage of the attacking force which should be lacking many if not all its spellcasters and can finish it off with spellcasters of its own would be wonderful & probably beyond the capability of AI programming (but that's what *should* happen).
Losing 15 mana a pop is huge. Teleport is limited to "controlled" areas as well, so you still have to build a strong enough army to do the job, which surprisingly easy in this game even if there were no teleport at all. You're not going to have the "world wide" coverage to teleport your entire strike force onto the Normandy beach which you alluded to, becacause you don't control anything on Normandy. You'll still have to fight for at least one town the old fashioned way. In this game, on a large map, to get the most out of teleport you'll still need to take cities on opposite ends of the map, you can't teleport in "uncontrolled" areas. I just use two different armies to do the same thing silmultaneously instead of moving only one "killer stack" to point A, conquering, teleporting to site C in between A & B (but as close as I can get to point and then moving off to attack B. It works out the same way in the end, but I conserve mana with my method.
In a multiplayer game, the guy that uses teleport a lot is going to be the guy that loses (assuming everything else is equal) because he will be out of mana all the time when he needs it (unless he "cheats" or "hacks" the code). The guy with multiple strong armies, a huge road network, and a fast "horse archer" reserve force that can quickly reinforce against any "teleport attack" will be the guy that wins because he'll conserve mana & use it when he really needs it, or better yet "bluff" or "feint" the other players into using up their mana via unecessary teleportation.
The way the game is now, there's an artificial cap to where mana is useful because you cannot regenerate fast enough. What does it matter if you have 300 mana wizard but currently only 30 essence? Are you going to sit around for 100 turns in an essence chapel (whatever the hell it's called) to regenerate 200 mana? I wouldn't. 15mana cost for teleportation, when you get nothing but movement in return, is not just worth it. You still have to fight for those cities. Teleport doesn't guarantee you'll get them or even win the ensuing battle. At least "firestorm" will help you win a battle and take that city, teleport is just a huge expensive spell that guarantees nothing in return; sure you'll be able to take the cities you plan on attacking, but not because teleport is "overpowered" rather because the AI is "too weak".