I think he’d had a massive case of culture shock. It’s not that things wouldn’t be graspable, but that so many fundamental things would be so completely different that the cumulative effect would be extreme disorientation.
nevertheless, I think that a robust individual would learn to cope. He wouldn’t worry about exactly how things worked – Electricity and gasoline engines and television and telephones and the internet would be “magic” or “unexplained technology” until understood. But our Roman friend would be bewildered and disorented by the following (NOT necessarily because they’re hard to understand, but becauise of how fundamentally different they are):
1.) Non-Roman Numerals and calculations
2.) Religion and how it fits into daily public life
3.) Social networking – whether you get together with people at the bar, the health club, organizations, or hang out at the mall, it’ll be vastly different from whatever he had (gambling in the forum, hanging out at the Baths, conversing in the communal toilet, whatever)
4.) Buying Food in sealed containers and getting meals at fast food/casual restaurants. – street-market food stands and the open-air market weren;t at all like this. Even if he goes to Rnglishtown Auction Sales in NJ or Haymarket in Boston, it’s not going to feel like home. And NOBODY here reclines to eat.
5.) I don’t think investing and the fundamentals of banking are going to weigh heavily on our time traveler until he’s been in town for a long time. Coins and purchasing will be pretty much the same, except that our are more regular and made of weird metals. He’s more likely to be freaked by all the paper money, but he’ll get over that pretty quick.
6.) CLOTHES, fer cryin’ out loud. Completely different in form, function, and price. We have nothing at all like a toga, and only barbarians wore pants.
7.) No Slaves. How can modern people even LIVE without basic servants? This one’s going to be a big mental re-alignment.
8.) PAPER is ubiquitous today. That’s going to be a bigger shock than computers. We don’t have news criers (a la Rome) or Tables of Laws up in the Forum, or costly vellum or papyrus books, but we have gazillions of cheap paper books and (for now, at least) newspapers.
Read L. Sprague de Camp’s classic Lest Darkness Fall, and invert the situation. His Martin Padway found that the things he expected to be real big changes were hard to implement, but commonplaces like Arabic Numerals, Double-Entry Bookkeeping, Paper, Distilled Beverages, and consistent soap were impressive achievements. Packaged food will blow our Roman away, as wi;ll many of our new foods (coffee, potatoes, corn, chocolate) and things that were rare that are now cheap and plentiful (sugar, good salt)