It would seem like a vastly different city. Actually, it would probably feel like it wasn't much of a city any more, more like a large town in comparison to the thriving imperial seat it used to be.

You would have seen a newly built city with fresh paint and eager people flocking to it. Emperor Valens had just defeated the Visigoths and trade flurished. New churches were being built and the wealth of the Eastern Roman Empire poured into the City.

Fast forward 1,000 years and you'll find a broken city. The black death had ravaged the city killing off half of the city. Previous to that, a civil war had torn the city apart and scattered the nobles across the country side. It had almost no projected power beyond the city walls and was almost ignored as a political power player at the time. Churches and important structures were being striped of their precious metals to sell off to pay debts.

The difference between the two cities would have been night and day.

J. Phillips, The Fourth Crusade and the Sack of Constantinople D. Nicolle, Constantinople 1453: The end of Byzantium, 32.