The first SC was a giant step forward for RTS gaming. At that time, all other RTS games were slow and plodding (just look at C&C:TS, released just a year later), and ideas like upgradable units, units with their own abilities, three sperate factions that all play differently (Creep was a BRILLIANT idea that really changed things up as far as base construction went) really made the first SC stand out.
The first SC was a revolutionary FPS. SC2 basically refines the forumula, although in this day and age, its nothing special. A solid FPS, but nothing more, but given the community behind the first SC even a decade after release, its no shock SC2 is getting such attention.