Starcraft is one of the best games ever to hit the market. There is no doubt that Blizzard has done an excellent game.
And then Starcraft: Brood War features a new ladder, single player campaigns, tilesets, SCX's, upgrades, and units. The Brood War ladder runs along side the existing one, so players without Brood War will only be able to compete in the current ladder. There are no new buildings; new units and their upgrades will be accessed through current buildings.