It's a very fun MOBA and the fact that the spotlight it got was to be a spot for free Overwatch skins during a rough phase in the game's life cycle is a travesty.
It's the MOBA with the most balanced roster of characters. There are definitely some characters that are better than others when it comes to the level of complexity versus the reward given by playing them, but nearly every character in this game is viable. HOTS also has the most unique roster mechanically; a majority of the characters you'd see in this game (Cho'Gall, Abathur, Stukov) would never be considered by Valve or Riot in the modern day.
The biggest downsides to HOTS would likely be the maps and Blizzard themselves. The maps in HOTS revolve much more around playing objectives than in other MOBAS, and some of the maps have grabbing objectives effectively making or breaking the game you're playing instead of giving a skill-based advantage. As a result, the gameplay on these maps can be jarring going from laning phase to a sudden objective phase, as giving up said objective would result in a huge disadvantage. Blizzard, in unsurprising tradition, sucked when it came to balancing this game when they were still actively updating it with being reliant on trickle-down balancing (making tweaks based purely on the pro-player level and hoping it'll change the game competitively and casually for the better).
Also Blizzard didn't nearly add enough Starcraft content which is at minimum 2 points docked.