Here's a very cool video guide as to how classic 80's synths were used to make the soundtrack to Stranger Things. Here's a more in-depth guide to the retro tech that was involved.
As mentioned, as much as I liked the series I think it started much stronger than it ended. I wouldn't say I was disappointed exactly, but for a show going in so many weird directions the last episode felt pretty conventional in comparison to the rest of the series. (The bit with the sheriff leaving some Eggos in the woods was great though.)
Then again, it was nice to have a tidy eight-show run that actually did manage to answer and resolve some of its own "big" questions with just enough left to the imagination for a second season.