The Dreamcast was ahead of its time as a "gaming console". They missed the mainstream due to weird ads and everyone wanting a DVD player (aka PS2).
In its short lifetime the Dreamcast got lots of games, well only about 210 in Europe, but it probably had the best "hits to number of games"-ratio. There was a lot of new things about the DC, e.g.:
the memory unit which added some to the gaming experience nice handheld ideas
the controller had analogue R L triggers first I think
it had online gaming and SEGA actually had a good going at it
the power of the DC was amazing for 1999 (just check Shenmue and compare it to other games at that time)
it had motion gaming which worked perfectly well (House of the Dead, Samba de Amigo (<- way better than the Wii version))
it had Seaman which let you control the game with your voice
games like Ikaruga, REZ, Skies of Arcadia, Grandia 2, Shenmue, etc.
Of course, part of the DC's charme and greatness is that it wasn't a great mainstream success which makes it seem awesome gaming-wise for some people instead of multimedia entertainment, and that's what gaming is about today. DC was pretty arcade, too, which is probably something special as well.
You really need to experience milestones in gaming to understand them. If someone hasn't played Ocarina of Time on N64 and played it now, he'd surely not say it's an amazing experience and the best game on earth.