EPTI've had extensive experience in LoL, DOTA2, and HoTS, with high competitive ranks in LoL and HoTS (after a thousand hours of DOTA I still get confused about what's happening on the screen). Over the past decade I've oscillated from game to game enjoying the highs and complaining to anyone who will listen about the lows; DOTA is mentally taxing to play constantly, playing the same map every game in LoL gets boring, and HoTS...well every time I stopped playing it for any extended period of time I had issues with the community.
Which brings me back to the article linked above. When players in LoL or DOTA play a ranked game at a reasonably high level, they will typically try to emulate professional play. I did not notice this same trend in ranked HoTS games in master/GM games I played.
For example, I found many players didn't understand getting value out of soaking an extra wave on BoE with a good poke/defensive comp holding 4v5 at the Immortal. It's common enough for professional solos to play a game of chicken in the top lane before rotating to the objective. To me, this seems like an obvious good move, especially if you're close to leveling, but in my experience I'd get flak from teammates if I stayed to soak an extra wave or if I told them to stay in their lane while we poke at our Immortal. I could understand complaining about the execution if I just derped and blindly farmed, but complaining about the concept of a clearly viable (and basic) strategic idea to grab an extra wave was offputting.
As the author of the article points out, the game was designed around QM as its core. It seems like the transition of ten assassins fighting to the death in mid at the start of the game to responsible, strategic play, just wasn't there. I never saw much in the way of innovation from players, which I'm sure is partly because the damage output of heroes in HoTS is comparatively lower to LoL and DOTA and partly because talent trees are not as versatile as buying items.
The small playerbase also caused me frustrations in the game. Ranked queues could feature players of wildly different skills. Whether a team has dead weight on their premade or matchmaking is slow in solo queue and gives you some bricks and some good players, it's never fun to lose playing a 4v5, and it isn't very satisfying to win a 5v4 when the enemy team gets the brick. This seemed to be an issue early in LoL as well (though it was more a disparity in skill, not rating), but was much more frustrating in HoTS.
A related issue is how hard it can be to carry a game of HoTS when you're teammates have flubbed the early game. Even if your teammates are decent players and just caught rough breaks early, your two kills in a solo lane probably won't snowball you into showing up at an objective and doing fed carry things. In addition, the outplay potential a single character has against several characters is comparatively low in the context of other MOBA's. To a competitive player it's frustrating to feel like you have (compared to alternative MOBA's) less control converting your great game into a win.
It's not like any of these things are singly damning about HoTS; and indeed there are a lot of great parts of the game: having multiple maps to play is refreshing, shared experience is interesting, and constant objective fights can create intriguing dynamics. But in order for a casual game to be fun over a long period of time for a wide audience, it needs high-level, innovative play to trickle down through the ranks. If it doesn't, competitive players become frustrated and leave for a more reliable experience elsewhere. When people who can pass the knowledge of competitive plays down to lower ranked players leave, the tools to make the game novel and enjoyable diminish. And when those tools disappear, viewership (people who are entertained and/or people who want to get better) declines and it makes it hard for Blizzard to justify paying every player's salary and footing the bill for all the tournaments.
