I realise that it's too late to start talking about this now (I exhausted my motivation for forum discussion in the months leading up to Oblivion) but I'll say my piece.
I play the thief in ES, I find that it's the best way to experience the world as you move through it slowly, examining every square inch as you go. It takes time and patience to pilfer anything of use but it's always satisfying when you succeed. The whole point of this class is to have freedom of movement through otherwise inhospitable and secure areas. If you make the world too accessible to other classes, what is the point of playing the thief, other than to roleplay an impotent hero?
Oblivion [censored] in the face of the thief. You couldn't sell anything you stole without having affiliation with the thieves guild and there was a daedric artefact, which anyone could obtain, used to open just about any lock, a giant 'f' you for those who'd spent the time working up the skill and collecting lock picks.
Lock picking should be THE skill for opening doors. Magic is convenient up to a level but it should not expect to be opening lvl 100 mechanisms, let's keep it this way. The grace of the mage is in its utility, not in its ability to do everything better than dedicated skillsets (invisibility is too powerful).
I don't like the idea of dragons being in an ES game but that's fine. The problem is that they're at the top of the hierarchy insofar as enemies are concerned. I hope they aren't easy to kill, I don't care how cool the fight is if a level 5 character can down them, which brings me to my next point. If you make everything easy to defeat, what is the point of playing the thief?
The thief sneaks because he/she needs to avoid confrontation, not because it WANTS to. Some guards should be more powerful than the lowest level dragons, it's about gameplay not 'realism'.
If people can do anything they want whenever they want and succeed, then the game will have failed because it offers no challenge. I understand the need to cater to all these Fable lovers but don't compromise the game any more than you have.
Restricted selling: fixed.
Skeleton key: It's a single player game. How is your character getting screwed? It's not like it's MP and someone using the skeleton key beat you to some goods. Just don't use the skeleton key. What they should have done with the skeleton key is make it magically regenerate if you break it say 10 x a day. So its a semi permanent lock pick, but if you abuse it with low skill you have to wait 24 hours before it can come back.
Lock picking THE skill? I don't think so. It should be THE skill to get in quietly and unnoticed, but mages should be able to get in too (making lots of light and some sound casting that spell and drawing attention), and warriors lock bash (wake up the neighborhood and possibly destroy goods in a locked chest)
I hope it's not too easy. They never said it would be easy doing what you want being who you want. They just said you can do it. I'm sure those who specialize will be able to do these things easier.