First of all, it's a really intriguing idea! How do people 'sail' the seas? Or are they pretty much impassable? I'm picturing explorers delving down the sides of the landmass lower and lower, finding relics and cities and unsavory beasties. Are there actual oceans? Because if you're going to have rivers on your mega-plateau they're gonna need something to drain into.

I would guess this land is colder on average since it sits much higher than the concealed lands below. Some questions to ask (that I can't answer by the way ) would be: Is the atmosphere thinner? If so, how has that affected life and the development of civilization? What latitude does the 'continent' sit at? Where do the prevailing winds come from? How do people build civilization(s) without an ocean? Historically, the first cities formed along rivers and sea shores - abundant fishing, ease of travel, etc. Why does the cloud cover never disperse?

It really is a cool idea, and I guess how realistic you intend it to be is going to inform how you develop it. You could opt for handwavium and say things are the way they are just because. Or you could go ultra-realistic and realize that some of the things you may have in mind for this setting just wouldn't work. Or some combination of the two.