I'm enjoying Skyrim but some fights are hard because they are cheap, not because they are hard.
A good stealth game is designed so that a smart player that knows how to apply his skill set can remain hidden through an unknown area and accomplish his objectives while introducing unforseeable but manageable obstacles along the way to increase difficulty.
The same principle should apply to a good combat game: a good player should be able to asses the encounter (one more more enemies? casters? melee? both? etc.) and develop an engagement strategy, engage and adapt (is the enemy casting spells? is he casting frost or fire? is he meleeing instead?) and then combine his skill set and resources in creative ways to overcome the challenge (this includes drinking potions and using scrolls).
A lot of times, I feel that your only option in Skyrim is to die a few times to get a feel for how to "game" the AI and then perform an overwhelming yet completely unsatisfactory victory by "gaming" said AI.
The root of this problem is the horribly simple AI that can only rely on spike damage to appear to be challenging. Even worse, the AI is so bad it doesn't even switch the type of spike damage it deals. Krosis is a good example of this simplicity. If he were smart, he would burn you down with frost if the PC is melee or lightning if the PC is a caster then switch to fire for the extra damage after you've been incapacitated. Instead, no matter what you are, he just runs back and shoots fireballs.
If the developers would have taken the first approach, they could have toned the damage down and make him hard because he is smart and keeps you incapacitated, instead of hard because he 2-shots you and your companion. Also, given the first approach with reduced damage, the player would have a bit more time to react and change strategies. The end result would be a fight that would be just as hard if not harder yet more satisfying to win because you won on your own merits, not on the limitations of the AI after dying a few times.
A well designed encounter should kill you when you fail to react and execute properly. Encounters can vary in difficulty by increasing the complexity of the response required by the player and reducing the marging of error on the execution of the player's strategy. Destroying the PC in two hits without warning does not involve a complex reaction or tight execution. It just means you have to die a few times before you realize you just have to run behind a pillar and spend the next 10 minutes pecking away at him with spells and arrows at your leisure.
I love all of the improvements Skyrim has made over Oblivion but I hope that for TES VI, Bethesda decides to focus on enemy AI and make a truly challenging game instead of a cheap one.