Single player wise, the three campaigns from a gameplay perspective are awesome. Varied objectives, interesting map design and allowing the player to decide which exclusive units to use, upgrades, etc. Quite a delight to play to be honest.
Story wise, it is awful. It doesn't even come close to Brood War; in fact, it undones many things that happens in the first Starcraft. Worse, it tries to do the same crap Blizzard has done since WC3: we must join forces for defeat an ancient evil. Amon is one pathetic excuse of a villain. Same with hybrids.
Multiplayer wise, in WoL and HotS, it basically involved: grab one or two expansions ASAP, amass army and have those death balls fight. Especially if you are a Protoss. What I don't like of SC2 is that reliance of units in an army. Back then, it was much more efficient to use the bio ball as a Terran than mechs or ships. Or the annoying Ling/Muta ball. And Protoss only used Colossis complemented with Stalkers and Immortals because other units weren't completely viable. (with the exception of High Templars).
Right now, I don't know the meta of the game. But I understand that in the competitive scene, the MOBAs completely destroyed SC2.
Co-Op missions were interesting with the commanders. But since it is a small pool of maps, it gets boring after a while.
And custom maps, which were quite important in Brood War and Warcraft 3, didn't grow in SC2.