Son of Godzilla said:
If you pretend Broodwar was Starcraft vanilla you would get completely fucking owned endlessly. Blizzard made Starcraft 2010. Is it a sequel? I unno.
If Starcraft 2 does not qualify as a sequel, than neither does 75% of other games that are called sequels.
You know, games like God of War II, Metroid Prime 2-3, Assassin's Creed II, Half-Life 2, Devil May Cry 3, Metal Gear Solids 2 - 4, Gears of War 2 and 3, Street Fighter IV, Sonics 2, 3, and Knuckles, Soul Caliburs II - IV, Halos 2 and 3, any Zelda game after OOT, etc etc etc etc etc. All of these games retain the absolute key assets of gameplay, story, characters, of their immediate predecessors. Some even have art assets that are ripped wholesale from those predecessors.
Starcraft 2 has a ridiculously upgraded Single Player, a completely different online service (for better or worse), numerous interface improvements that change the pacing and accessibility of the game, with numerous changes in unit composition, tech choices, and play styles.
If Starcraft 2 can not properly be called a sequel to the original, then you have to levy this exact same complaint against the vast majority of games that are called sequels.