Chen Stormstout was (as far as I know) the only Pandaren known generally before this expansion. I remember the Lich King making plenty of cameos throughout levelling in the Wrath of the Lich King expansion. Does Chen make any cameos while levelling in Mists?
Yes, he does. You can find him in the Valley of the Four Winds, where he has his own quest line. Among other things:
He is now accompanied by his annoying niece, Li Li, making the duo suspiciously like another uncle-niece team from another popular Blizzard game.