I'm not sure if you're saying that the Minidisc couldn't be slot loaded - it could- or that it didn't take off in cars. Minidiscs were used heavily by the theater and pro audio industries for years (2010 is when I saw the last Minidisc player in theater racks) because they were so much tougher than a CD and could be edited on the player itself. CD Players and CDs can't take much abuse. Most of the pro audio playback is now solid state devices (for people who can afford it) or laptops (for lower budget minded folks).

Most wireless microphones operate in the "white space" frequency ranges. The FCC pushed wireless users out of the 698-806 MHz a couple years ago and caused havoc in the theater and concert industries - the small theater I worked for spent over thirty thousand dollars replacing their wireless mics, because it is now illegal to buy, sell, or use a 700 MHz microphone. I can't imagine what it will be like if they take away all the spectrum. It's hard enough as is to do frequency coordination for twenty or thirty mics that are used in larger productions. More info here: http://www.shure.com/americas/news-events/spectrum-issues/white-spaces/index.htm

It on the front page of the New York Times...

I work in a theater. We will have to spend between $6,000 and $10,000 to replace wireless microphones in the 700 MHz range, and there's no guarantee the new ones will work anymore, due to interference. If the deadline was extended, we would be able to use the old microphones for the last show of the year and wait to see what happens during the summer, which is an off season. I think that DTV is good in the long-term, but they could have done it without screwing over wireless microphone users (a high profile industry-- think of all the rock stars and football stadiums).

MiniDisc is often used in theater because you can edit tracks on the disc without a computer. I use it to make back-up recordings and play sound effects simply because it's easier to use than CDs.