I asked a bunch of teenagers what most surprised them at the MAGFest pop-up arcade, where dozens of games of all stripe were right there ready to be played. To a person, it was one thing:
The vector monitor on Asteroids.
If you see Asteroids online, the "screen" probably looks like this:
That is, it's a JPEG or PNG or whatever, with the source likely being an emulator of some sort. So everything is crisp, and rasterized, and generated as if were a screen map, that is, from the assumption that it's a raster screen. Playing the emulator itself is similar -- there it is on your recently-vintaged flatscreen, perfectly sharp, definitely of the modern desktop era.
When you see it in the arcade, an original Asteroids machine screen looks kind of like this:
The vector lines, which are created by aiming a beam DIRECTLY AT YOUR EYES only to be stopped by a coated piece of glass, have a completely different feel. The phosphor glows, the shots look like small stars floating across the glass, and a raster line is not to be seen. It's an entirely different experience, and the teenagers at MAGfest had never seen it before, and unfortunately, it is well on its way out.
I am very glad to hear about HSLMAME and that the MAME folks are finally starting to take monitor-hardware-emulation seriously!
There are some interesting links in the comments over there, too.