Well they didn't bring over the entire game. It's just deathmatch and only the 'classic' maps that were played so much when Quake was at the top of the heap. The maps that are included in DMC are nearly play and look identical to the originals. The maps that will be included in the first release are:

DM2 - Claustrophobopolis	[also a snapshot from original]
DM3 [2] - The Abandoned Base	[also a snapshot from original]
DM4 - The Bad Place	[also a snapshot from original]
DM6 - The Dark Zone	[also a snapshot from original]
E1M2 - Castle Of The Damned	[also a snapshot from original]