starcraft was amazing in its time when the only rts out there was like warcraft II, Dune, im not even sure if age of empires and command and conquer were out yet....
starcraft I missions, and brood war the expansion, were very repetitive : (build a base, trash the opponents base). So the gameplay didn't vary much, but the music graphics (for the time) and story were and still are epic.
in sc2 you get a modern amazing rts, so many varieties of missions while still being faithfull to the original, im not done yet but so far im really impressed entertained and ...... brutalized (for playing the campaign on brutal)
Not played any Starcraft but if you want a great RTS with a difference try Company of Heroes