A Viagra advertisement.
But most people weren’t in the know. If you had a PC in your room in 1978 you were a very rare bird indeed. Personal computers were exceedingly unusual until the 80s, and it wasn’t commonplace to have one until much later. Using it for external communications was a rarity as well.
I’d think our customer from 1978 might just be exceedingly pleased that a nuclear war hadn’t taken place. Certainly, the idea that the USSR had just sort of collapsed on its own, without a massive war, would be a mind-blowing concept. Imagine if you zipped ahead thirty years and found out that, with relatively little fuss, the United States had dissolved.
I think the time traveller would be shocked at our two recent wars in Iraq (and Afghanistan for good measure). Not necessarily saying for good or bad, but I think in 1978 the ver idea of another extended overseas military engagement–and an open-ended one at that–would have been unthinkable. Also the whole global hegemon thing, with no Soviet Union to get in our way.
Also they’d probably be very surprised at how well the American economy has done, with at least two major extended booms since their day. Back in '78, malaise was the watchword and (as far as I remember) you would have thought that everybody felt America sucked.
I remember how excited people were when the Dow hit 2,500 in 1987. In the last year it’s lost that much and it’s still more than four times higher than it was then.
A 1978 person wouldn’t be savvy enough to think something looked “too CGI”. People might say now that Terminator 2 or Starship Troopers looks too CGI, but I think that most people were blown away at the time.
However, I agree with you that movies have always been convincing to the audiences of the time. My nomination is local TV commercials. Even small cities probably have companies that can whip up a nice (and affordable!) commercial for a local business, with CGI effects. In 1978, the best that could be hoped for was probably poorly-done green-screen.
How do you impress your time-traveling visitor from 1978?
Take them with me to 2048. Duh.
And it only costs 40 times as much. 
I’d take a snapshot of my visitor with my digital camera, download it to my laptop computer, and use my Wacom graphics tablet to easily superimpose his or her face onto the body of, say, a tiger.
Then I’d tell them who the governor of California is.
Barack Obama
Full Metal Lotus is cool. I had to say that. 
•For the record, I would have to show the time traveler the Star Wars prequels to find out if they thought they sucked, even in comparison to the original.
•Maybe try and find some medicated and/or otherwise now fully functional people with mental illnesses for the TT to speak with (Granted, '78 wasn’t the Walter Freeman dark ages of psychiatric treatment, but it’s still been awhile).
•Disney World might be an idea, if I want to impress. It’s not completely beyond what the world of 1978 could produce, but hey…frikkin’ Disney World. It’ll work. 
•Take the TT to an antique/junk store. Watch the reactions ranging from wistful nostalgia, to quiet bewilderment at old, broken devices that they can’t even identify.
•Try to find some Log Cabin Republicans to debate if Powell or Rice was/has been the better Secretary of State.
So many great ones already. I would want to add a few things though.
I would explain Lasik, show how Solar Panels really work domestically now and as part of the electrical grid. Scientific America on line should impress a time traveler if they are at all a Science Geek. If they are a Baseball Geek at all the unbelievable amount of free access to stats on line would completely blow them away.
First, Pittsburgh. Seriously. That place was a pit in 1978, and it’s a reasonably clean city now, with less pollution that many others its size.
The graphics in modern computer games. As an old fogey that wrote graphics software in the 70s, I’m still impressed with World of Warcraft. But games like Tabula Rasa and GTA4 are really amazing! Wow.
Handheld GPS units. Well, handheld almost anything.
Personal stereos. My 1978 stereo had speakers that weighed 70 pounds each and were three feet tall. The stereo system itself took up a whole rack. The sound quality we can get today from a handheld MP3 player connected to a pair of book-sized speakers and a subwoofer the size of a shoebox would blow their minds.
LCD and plasma screens.
Special effects in movies. What we can do with CGI is amazing. Heck, just the TV commercials are amazing. Even non-CGI special effects. Show someone from 1978 the Matrix effect (freeze action in midair, move camera 180 degrees or more, then resume action) and ask them how it was done (it could be done using 1978 technology, but nobody had thought of it).
A lot of things would depend on the person. If it was somebody in the publishing biz, I’d walk them through Photoshop and InDesign. Medical tech? I’d explain the monoclonal antibody treatment I received when I had cancer–it’s mainstream now! Photography? Modern digital cameras are just amazing. Teaching? Even elementary classrooms have “smart” whiteboards and good projection systems, and what we’re doing with distance learning would blow their minds.
Show them how far Dungeons and Dragons has come with the plethora of RPG games on my souped up Dell 720…and other board games gone digital.
For all those saying “computer,” I’d specifically show a 24" iMac. I’m from this time and looking at one still puts me in awe. It does the same things as a Dell or HP but it just looks sooooo futuristic and cool. Huge face, slim profile, aluminum and black. I’d make sure it has the wireless keyboard and mouse, too. Just adds to the Star Trek vibe.
I’d also bring them to WalMart at midnight or 2am just to show how much easier life can be in the future. No waiting until 10am or rushing to get there before 6pm.* No standing in line at the bank to get cash or writing a check at the store. Any time, without cash, pick up groceries, clothes, gadgets, hardware. Zip through the scanners at checkout, pay with credit/debit card and out the door!
Before we left, he’d have to stop by the john to use the automatic flush toilet or urinal and the touchless faucet. Bathroom fixtures smart enough to turn on and off by themselves, that has to count for something!
- North America visitor only.
This phrase:
Maybe I was an uninformed kid at the time (maybe?), but I got the distinct impression growing up that most people thought of “cancer” as an automatic death sentence, or at least incredibly serious and tragic. I think the fact that there are a heckuvalot of people who “had” cancer and no longer do would be pretty mindblowing (at least to my eight-year-old self).
I’ve got to go with stores selling organic vegetables that aren’t bruised and covered with flies.
Would a visitor from 1978 even understand the modern definition of organic produce? I have vague memories of “pesticide free” produce, but I don’t recall organic until I was in HS. And it was a common complaint at my family’s dinner table because my parents both thought it was a stupid term.
If they were eating it, sure. It might have been called “all natural” in some places, but in 1979 I was buying organic vegetables at a “health food store.”
As a former denizen of 1978, my reaction would have been that 2007 really screwed up the porn. Back then we had Flesh Gordon, Debbie Does Dallas, Debbie Duz Dishes, and a whole host of well produced and almost amusing porn. Today we got …? Pirates, I suppose.
Feh. By 1978 computers shrunk from mainframe size to LSI-11 size. In 1975 I saw a PDP-8 in a small suitcase that a professor had put together. My adviser ran a conference on the '80s just about then. One of the participants was Adam Osbourne, who got portables exactly right (this was before he started his company.) I’d think today’s machines were cool, but wouldn’t be amazed.
I went to a 24 hour grocery in Boston in 1972. yawn.
In 1980 I ran into an automatic flush urinal in Austria, and not in a high tech place. It was fun - all the computer scientists at the conference I was at went into the john to play with it, but it shows that toilet technology was right up there.
By 1978 I had already used:
Instant messaging
Chat rooms
Message boards
Been to a semi-pornographic “website” - actually a Plato lesson.
Played interactive games with people around the world.
Been to a MUD.
Used a plasma screen.
Stuff I would have been more impressed with are CDs and DVDs. I would have killed to get some of my favorite movies to watch at home.
And spam. That they let non-programmers on computers to send spam would have made me wondered if the web was a good thing.