Brutal difficulty on single player in WoL is incredibly challenging and some of the Mastery achievements for Hard difficulty or higher in HotS single player are very challenging. The campaign itself is very well done and they've evidently learned from some of their mistakes in WoL, upgrades are actually meaningful and you get more bang for your buck from them and they brought back the Hero Unit system from Warcraft 3 which really makes you feel like you're a badass.

As for multiplayer, if the end of beta events and the current HotS launch events are anything to go by then people are playing far more aggressively rather than 4 base turtle tactics.

This is also ignoring the fastly growing Arcade scene and the massive overhauls they made to versus AI in HotS.