A few years ago I hurt one of my shoulders pretty badly, so I could barely move and had to sleep in a sitting / reclined position for almost a week. No keyboards, no computers.

Boy oh boy did that LotR premium extended something-something edition on DVD turn out to be a godsend with the countless hours of behind the scenes footage and interviews, and director's commentary. I watched it all.

Did you find the easter egg with the Elijah Wood prank interview? That's the kind of stuff modern media lacks.

One of my first DVD experiences was the original Blade film, and it had an alternate ending available to play.

That blew my mind and was an awesome experience. In the age of dial-up internet it wasn't like you could find and / or watch that online.

DVD supports an absolute amazing amount of features many of which were rarely if ever used, and some never implemented on most players, likely.

It's why the original Matrix DVD dual layer release was so hard to duplicate / replicate by pirates/"enthusiasts" in a way that would fit on single layer DVDs. One of those was a secret "follow the white rabbit" mode that lead to hidden content.

doom9 had reams of guides dedicate to the matter: https://forum.doom9.org/showthread.php?p=313722

What's even more amazing/sad is that Matroska containers support much of it, but no one has implemented a player that has all of them

I'd rather have extra content in separate files so it's playable by anything.

Unlikely. All players manufactured had to pass DVD specification testing. This is not to say that there were not hardware bugs in early manufacturing runs, but the realities of shipping hardware players prior to widespread, cheap and consumer-friendly firmware update methods meant there were far fewer bugs and hardly any showstoppers. The limiting factor for amazing presentation features was and is authoring platform support, and it's astonishing how many older authoring platforms supported these features even though their newer updated versions dropped support for them (e.g. karaoke-style subtitles in Maestro lost in DVD Studio Pro... and don't get me started on support for seamless story branching). Player devices and commercial software had to support all the features to sport an official DVD logo, and ones that didn't (or did and failed to pay the patent licensor) were sued out of existence.

Yeah, early on in the DVD era having deleted scenes and commentary tracks was totally new and a big deal for a lot of us.

In the peak of the DVD era my SO and I were both big movie fans, and at one point had watched literally everything the Blockbuster and Hollywood video in our neighborhood had. There just wasn't the embarrassment of riches of the post youtube world.