I'd like to say at this point I'm strongly against anything which forces you into completing the MQ. I imagine that is the sort of thing most people will think of as they read the title of this thread, and that is not what I'm arguing for. So, for me, areas of land you unlock by advancing through the MQ is a definite No, with a capital N.
However, areas requiring a quest or series I would be in favor of. Imagine this; a plague is spreading across the land. In an attempt to seal out the disease, the great city of Solitude has closed it's gates to the outside world. No-one is allowed in or out without the correct papers, certifying their health. Say the player suddenly finds themselves requiring access to the city, but the guards refuse to permit them. And thus we have a quest. Steal some papers, acquire them legally, bribe some guards, sneak in over the walls... multiple ways that quest could end. And once you have completed it, you are rewarded with access to a brand new area.
On a smaller scale, unlockable areas could add a lot of replayability and culture. Clubhouses accessible only to orcs, inns refusing admittance to Khajiit, parties only attendable by the very wealthy and underwater hideouts requiring an argonian to discover.
I'd love to hear your thoughts on the matter.