Just a possibility, but here's some food for thought towards making an in-depth skill system:

Not to seem like this would make life difficult for everyone, (because it wouldn't), or just another super-hard-core-rpg way of doing something; but here's an idea for how skills could develop:

In Morrowind there were three armor branches, (light, medium, and heavy), these were simplified for Oblivion into light and heavy. What if the exp you would be putting toward light armor, was instead differed to the armor type...i.e: leather or steel? No categories, just skill toward a specific set.

Then in this case, you would start the game with your simple leather armor and progressively improve it the entire time you play. By end-game, the foes you're fighting haven't been using the rare "daedric" or "glass" stuff for nearly as many hours as you have, so you can stand toe-to-toe with pretty much anyone. The same set of leather you began playing with on day 1 will still be useful - if you maintain your equipment.

On the contrary, the incentive to 'level up' from the lesser tiers for some classy piece of daedric is still there. IF you use the thing, you will improve...it could even improve with you...lets say the soul that is bound to it learns as you learn. (I'm getting side-tracked, but that's also an idea) If you played with a (higher) tier 2 armor type, the same amount as a (lesser) tier 1, your skill with both being equal, then the tier 2 is slightly more powerful. (You can take a bit more damage with it)

Just to be clear, the curve would level-off somewhere, making players choose between leaving their weathered, battle-scarred and proven favorites for a new and shiny toy. More people would be seen late-game wearing early-game-looking gear. Simply because they would prefer not wasting the time they've spent skilling-up the lesser tier. Not to say you can't improve upon something through crafting...you should be able to turn blacksmithing and enchanting into an art form on a scrap of leather if you're proficient enough.

But, if you just earned a brand new set of the highest level gear in some raid, you still have room to grow. Your giant-metal-harbringer-of-death-power-suit may not be invincible, yet, to the little guy jumping around in bubble-wrap...but given a long enough timeline...he might wish he'd upgraded. This would help make the game more enjoyable for everyone - there's pay-off for the guy who slaves himself away grinding gear, and there's balance for the guy who plays casually with older equipment.

Thoughts?