There were lots of differences. Here's what sticks out the most more than 20 years later.

Dortmund is a large cosmopolitan city in the former West Germany. My friend was on exchange at a Gymnasium[2] so most of the kids he hung out with were college-bound.

I was in a town (small enough that I'll omit the name) that was about as rural as you get in the former East Germany. I literally met every 15-19 year old that lived in the town. I don't know if they nearly all showed up to the parties normally, or if it was just because an American was there.

I picked up a very strong accent while I was there; most noticeably the "ch", which, in "proper" German is a breathy sound made from the back of the throat, was pronounced more or less like the english "sh" sound. Also the "u" sound is pronounced much more closely to the ü sound; to the point where I couldn't tell the difference at conversational speeds (but I did learn to speak them differently: schwul is a not particularly nice slang term for homosexual, schwül means muggy).

In Thueringen, the schools were very heavily tracked[1] from a very young age, and since there was no gymnasium[2] in the town, there was only one teenager in the entire town planning to go to University. After a week of me living in Germany, she was the only person I met who spoke English significantly better than I spoke German (and no, I didn't speak German very well, which caused lots of communication problems).

In Dortmund, my friend was commonly asked "Why are you learning German? Everyone in Germany speaks English just fine!" Even when I visited areas closer to Erfurt (the capital of the Thueringen region), I often got "Gott sei dank, du kannst Deutsch" (Thank God, you can speak German). This was exaggerated by the fact that even well-educated adults would have been encouraged to learn Russian as their primary foreign language rather than English in the DDR.

At the time, there was also a lot of social tension between the West and East. In Thueringen, it was a complicated relationship, because a lot of people had a sudden windfall when their DDR Marks were converted to Deutchemarks on a 2:1 basis (much better than the open-market rate). They were thankful for the West doing this, but also felt like the Wessit (westerners) looked down on them as lesser and in need of improving.

On my flight back to the US, I sat next to someone from the Frankfurt area, and once I mentioned where I had been living, he went on about how the Ossit (Easterners) were lazy, but it's not their fault because they were raised under a regime where work was not rewarded &c. My friend who went to Dortmund confirmed that this was a not uncommon thing to hear when asking people what they felt about the reunification[3]

I am by no means conservative, but to this day I get the same icky feeling when I hear coastal Democrats say similarly condescending things about poor red state voters voting "against their own interests."

1: https://en.wikipedia.org/wiki/Tracking_(education)

2: https://en.wikipedia.org/wiki/Gymnasium_(Germany)

3: German reunification, the upcoming Euro currency, and the ongoing World Cup (until Germany lost to Croatia) were some suggested conversation starters to get us speaking German more, so both my friend and I asked people about those things.