It's like you read what I wrote, but you didn't. Diablo 3 was the last major game released by Blizzard and reviewers were given early access for reviewing purposes. The game got lots of 9.0+ before release which generated in massive hype, which killed servers, which pissed off a ton of customers.
Compared to Diablo 3, HotS launch has been REALLY low key. I haven't seen a single TV ad, youtube marketing has been relatively low (only a few hype videos and a trailer), and overall it just feels like Bliz wanted to keep HotS low key at launch to try and not kill their servers.