Is it the gigantic, sparse levels made out of 10 polygons per square mile?
I only played the game for a few months 27 years ago but it has stuck with me. I don't know if I've ever found a game that was that compelling and fun. But then again, I haven't really given any of the modern games a shot and mostly gave up with the FPS genre after team fortress classic fell out of favor
The Descent 2 levels were indeed 'huge' in a sense, there were vast spaces and tunnels and mazes - and indeed, arenas - in the initial releases of the game, per my own experience, of course. The first days of Descent 2 installs were, for me and my colleagues, really good reasons to go to work. :)
Anyway yeah, 10 polygons per 'mile' seemed about right. The Descent engine, I think, was well exploited in the sense that artistically, few polygons ended up being enough..
I often have flashbacks of the tight, cramped death tunnels and the inevitable race to the Fusion and Gauss Cannons... a colleague once had the Earthshaker warnings in a loop, and it literally immediately changed the room temperature whenever someone called them.
Anyway, apropos classics, in favour. I think these games are pretty much gems.
Tribes 2 was basically a reimplementation of the major Internet protocols at the time... IRC for their chat rooms, newsgroups for the forums postings, profile pages was kindof proto-myspace, joining a "tribe" had obvious Unix groups parallels.
...but yes, MA[2] has now been "in development" for nearing a decade now, and there is still activity with the original clients/servers most day of the week.
I still occasionally have dreams of various Tribes levels.
Same with Descent - I swear there is an alternative universe where my soul is adrift in that space, recently ejected from my ship ..