The physics simulation is on just the right side of fun/realism, insane contraptions work but the system is flexible enough that players jury rigged turrets.
Lots of games in a similar vein have followed, but I haven't found any to be as easy to play and as fun.
My favorite creation was the "wall-driving car". You could just make a car, and put a propeller on top pointed upward, and it would press you against whatever surface you were on so you could drive on walls or even the ceiling. That worked. And I'm almost certain the devs never imagined it.
I also made a working pod-racer by putting tiny propellers underneath a pair of jet engines (and under the seat), and connecting them to the seat using those flexible coupling-beams. And it drove like a pod-racer. It looked exactly like something out of the film, the way it moved. It was amazing.
Maybe I need to boot that game back up :)
Starsiege: Tribes
For context: https://m.youtube.com/watch?v=zIAWzG-wOKY&t=4m11s (~1999, probably)
The core feature of a playable, entertaining emergent game is a set of flexible gameplay rules, able to bend and tolerate the unexpected without breaking.
In the case of Tribes, there weren't any gameplay rules predicated on movement speed or height. Consequently, skiing only enriched gameplay. I would argue that EVE Online features the same style of emergent-tolerant rules.
In contrast would be something like Braid, where the available actions and rules, while seeming free, are in fact extremely rigid and intolerant of the unexpected.
Failed emergent games typically fail by innovating in their gameplay, but attempting to apply inflexible or overly-specific rules on top of that.
That said, HN probably beats the average on finding those folks too. ;)
Tbh, I expect most of the downvotes are from people who weren't gaming in the late 90s.