VGA support, hand-drawn (not rotoscoped) high-res sprites and backgrounds with gorgeous colors, still one of the best-looking 2D fighting games around--the later GGXX games got weirdly scaled, with lots of jaggies--and their combat systems got more complicated.
Important stuff: GameFAQs says beating Testament (lvl 20) and Dizzy (lvl 30) in Survivor mode unlocks them for play.
More character colors: select with Start - color 5 X+A - color 6 Y+B - color 7 and more: Dark - beat Survivor, hold Start 2 seconds when selecting Gold - hold R trigger, press & hold Y, hold both until match starts
Survivor mode is pretty neat--quite hard of course, but you constantly face special, specially colored versions of the regular cast, so it really feels like an adventure mode.
Ack, those codes for extra colors appear to be bogus. According to the GGX DC color FAQ https://gamefaqs.gamespot.com/dreamcast/340773-guilty-gear-x/faqs/18563 there *are* 10 colors per character, but the second set of 5 are all unlocked by beating Survival (level 100--and it makes it sound like you only have to do this with a single character); once you've done that, apparently they're accessed by pressing one of the five regular buttons (ABXYStart), but holding it for a few seconds instead of just pressing it momentarily.
I haven't beat Survival mode of course so I can't verify...and likely never will be able to. ; ) ;_; Okay okay I just need to GET GOOD! ^_^_