Comparing Brood War to SCII is kind of unfair, in my opinion. Much of what drives people to say so is mere nostalgia. Starcraft had a novelty that its successor could never hope to compete with. It was the definitive RTS game in the era when RTS games were still innovative and novel. It spawned a multitude of immitators, and for a while, it became a major genre. Now, the RTS title is a niche game market, having been positively overtaken by more simplistic MOBA titles, which eschew the base management and strategy aspects in favor of more accessible multiplayer.
Also, it's worth mentioning that Starcraft isn't DONE, in the same way that Brood War was for most of its played life. I think the decision to spread Starcraft across three story expansions was a regretable one, because the designers must necessarily leave niches open in their unit design to fill in later expansions. Remember that SC:BW was released less than a year after the original game. Which meant that for the rest of the lifetime of the game, the iteration of balance was approached with with a finished palette of units.
Sadly, I also think that being the successor to a franchise has stifled what the game could be, as the design must fall within our pre-conceptions of what the game should be. If Blizzard removed iconic units like the Templar, Marine, Hydralisk or Siege Tank from the unit lineup, there's be a great hue and cry, yet that very constraint limits the kinds of novelty the game can offer. That fact is going to always make the current offering pale in the shadow of its forebearer.