Limiting the EXP gained from lower end items would be a good way to do it.
That won't really help, for two reasons:
1) The XP needed per skill increase is sufficiently low at low skill levels that you gain access to tier-2 materials very quickly, even without things like dagger-spam. This means you won't be making lower-end items for very long, thus negating the limitation.
2) There is a large number of uncontested ore veins, from which you can mine a massive amount of materials that will let you power through the skill gains even while making nothing but cuirasses. These veins cover the entire range of metallic materials, so you'll always have something you can use to gain more skill.
Wrong; their job is to make a fun game. Granted, they try to motivate the players to play the way they designed it, to prevent something from being exploited or abused is nowhere near their job description; especially when they design a single-player sandbox game.
Actually it
is their job to make sure the game is balanced, as part of proper game design is making sure that even the most heavily-geared characters can still be challenged in combat. Whether that game is single-player or multi-player is
completely irrelevant, since in both cases an unbalanced game will ruin the experience for a (potentially large) portion of the player base.
Now, the crafting skills in
Skyrim (Smithing, Enchanting, and Alchemy) are not, in and of themselves, unbalanced, nor, necessarily, is combining two or even all three of them. What
is unbalanced, though, is the fact that characters who
do stack crafting skills exceed the top end of the difficulty scale by at least an order of magnitude, because something like that
should never be possible in a properly balanced game. That is
not to say that one should not be able to become incredibly powerful via use of crafting skills, since that's almost their entire point, it's that the game should calibrate the difficulty of encounters to suit the power level of the character.
In other words: if I craft an uberized Orcish Greatsword that, after adding armor enchantments, is doing 562 damage per strike without any damage enchantments on it, then the game should react accordingly and provide suitable opponents. This can also apply to armor, which would provide a good reason to over-stack AR in order to ablate Piercing attacks, such as mace-wielders with the associated perk (there are a few of these already, but their crappy weapons tank their output). The game
doesn't do this, though, with the result that use of even one crafting skill can break combat wide open.
Crud, missed a quote.
@Dragon Bait: yeah, that probably would be the topic, although I suspect it wouldn't be quite as contentious as there tends to be more agreement on that front.