In the DICE Summit, Todd introduced a diagram illustrating the sweet spot between Boredom and Frustration that designers aim for when designing gameplay.
Using it, I think I can illustrate just why Destruction missed the sweet spot: http://img607.imageshack.us/img607/1073/boredomfrustration.png
Without impact, you have no means to stagger your foes with Destruction. While you can walk up to someone and stagger them with a power attack with a melee weapon, you can't with just straight Destruction Magic. Melee foes can just waltz right up to you without a hitch to start hacking at you. FRUSTRATION!!!
With Impact? You can literally stunlock a DRAGON until you kill it. And Stunlock is universally decried as a cheap move in any sort of PVP environment... and so it is here too. Boredom...
Cast costs? You're pretty much tapped out on your magicka bar trying to kill anything beyond a low-level rube mook, and oftentimes they're still standing, or you're stuck using the Novice/Apprentice spells because they don't have the abhorrent damage/cast cost inefficiencies of anything Adept or above and complete lack of depleted magicka recources (which melee weapons have in normal attacks and archery never has to worry about because of the prolific availability of arrows). To kill anything that you couldn't just kill by looking cross-eyed at, you're required to carry around huge piles of Restore Magicka potions, since the agonizingly slow 66% hit to your magicka regen rate in combat makes relying on that, even with fortify enchants and potions, a futile effort. FRUSTRATION!!!
On the opposite side, how do you fix that? Enchantment abuse. You coat yourself head to toe in Fortify Destruction enchants to make some effort to mitigate the hobbling cast costs of anything beyond the Apprentice spells so you can actually kill something without 20, 30+ casts of Firebolt or its contemporaries (and as a result become a one
The root cause of much of this is the combat regen penalty IMO. It's a massive limitation on a kill skill that is completely reliant on a very limited resource to function (unlike melee and archery as discussed above). Many of the other schools of magic aren't quite as affected by this as Destruction is, because their effects are usually over a duration. But with Destruction? They're instant... and aren't really that great compared to the many other things you can do in other schools. If we could actually use the Adept and above spells more than a few times before we're tapped out, the Impact problem wouldn't be an issue because we wouldn't NEED stunlock (Impact needs to be nerfed of that regardless). And also, enchantment abuse would no longer be de facto mandatory to be able to get use out of Destruction. There are many a PC mod out there now that significantly increases the combat regen rate for magicka. I'm using one, and I actually can use Destruction as a viable kill skill because of it without resorting to the boredom-ifying workarounds available in a vanilla game.
