Instead of a new game, it's more like a fan-made expansion pack. This is because the current generation of Blizzard employees never worked on the original and lack the creativity of their predecessors. Instead, Blizzard hired obsessive nerds that eat, live, and breathe Starcraft to design the sequel so naturally they couldn't come up with anything except duplicating the original game.
"Hey man have you played SC2 yet?"
"Yeah, I tried it out but it's the same freaking game I played a decade ago. Coulda saved 50 bucks by dusting off my Brood War CD, oh well."
Hardcore basement-dwelling nerds complain about Starcraft 2 for the exact opposite reason: "OMG HOW DARE THEY CHANGE ANY OF THE UNITS OR IMPROVE THE INTERFACE WHEN I SPENT THE LAST 12 YEARS OF MY LIFE PRACTICING HOW TO MAKE SCVs GO TO MINERALS AS SOON AS THEY SPAWN!!!11!"