Thanks for starting the thread!
I think at first glance, Marty would be a little disappointed that the future isn’t as futuristic as he was probably hoping. Cars still look like cars, clothing styles are different but they still work the same way (no self lacing shoes or auto drying jackets). Hill Valley is fictional, but if it was a typical smallish town from 1985 he’d probably be saddened that most of the stores would now be shuttered up having lost business to mega stores. However, Hill Valley appears to be more of a suburb, so it would probably be significantly bigger and busier than he was expecting which might throw Marty for a loop.
Back in 1985 Japan seemed poised to take over the US, according to Marty “They make all the best stuff”. Sony was such a big deal in 1985 with the Walkman which was so revolutionary that Marty used it in 1955 to show how futuristic he was. But now we have the iPod, and it’s made by Apple. Apple! The computer people!
The biggest deal for him would be smart phones and tablets. That one tiny device can call from anywhere, text, use GPS, access dictionaries and encyclopedias, take pictures, record video, watch TV, play music (and at better quality than his walkman!) play games, and behave as a computer would be beyond amazing. Remember how revolutionary the camcorder was in the first film? Now that fits in his pocket. Remember how advanced computers seemed in 1985? More power than that fits in that same pocket. Video phones are practical. Books and magazines (interactive!) all fit on those same devices. GPS - the idea that the device knows exactly where you are and can tell you where to go! Flat screens! The internet, at first glance, would probably not grab his attention too much because I think it’s too big to take in at a glance. He’d be amazed at how connected things are, but I don’t think he’d comprehend just how influential that connection truly is in our world. And as mentioned above, the video games would probably grab his attention so thoroughly that I’m not sure Marty would leave the store.
Thugs and bullies were more visible in 1985 hence the character of Biff. The movie carried that into the future but I think Marty would be surprised at the relative lack of “bad kids”. This might be my own bias (I was 13 in 1985) but I think the push against bullying has pushed that activity into darker corners - something that Marty just walking around wouldn’t see directly unless he stayed here for any length of time.