For me it is the unit interactions

Whoops, this got longer than I intended but just keep in mind that I haven't played LotV.

I don't think the balanced was the issue for SC2, maybe I am exaggerating but BW had some broken shit (eg. siege tanks, storm, dark swarm). However the interaction of the other players broken shit made it more fun imo. In SC2 I feel like there is something that is missing there, most spells are generally bad and multicast in my mind does not make up for the fact that the spells are weak. Also the heavy counter mentality in the game, makes the units less interesting since you can know just by looking at two armies that the other will win.

Strategywise there is alot more depth or possibilities in BW, I mean you can't as a Zerg play aggressive against protoss or terran and not being all-in in SC2. Sure playing super cheesy made your follow up slower in BW however if you did the intended damage you could still continue the game (and create some timing windows for you later). So the main issue is not in my mind that every mid-game is the same for each SC2 game. It is that every early,mid and lategame has the same feel and has the same type of build. At least compared to BW where crazy shit could happen in the early game and the midgame was normal, and that was not at all uncommon. True that every opener in SC2 is not the same however they are all in the same category, i.e mostly greedy (expanding). The fact that aggressive openings are not rewarded really hurts SC2, it keeps the greed real.

As for gameplay or control of units and unit behavior there is pros and cons in SC2. The pros is definetly easier to manage bases eg. multi-building hotkeys which I think is fine for me and "unlimited" control groups are fine. However the "micro" part and especially the air unit control is in my mind not as mellifluous as in BW. And the auto-targeting and multi-casting is bad for the game (yes I think there is difference between building and unit management) because it takes away the micro aspect from the players which in turn hurt those who can focus on micro or those that focus more on their micro then their macro. The lack of control needed for macro is one of the frustrating things aswell that is really hindering low-skilled players and beginners, whereas the micro aspect is something you can train yourself if you really want to be good at the game.

I played WoL alot but stopped after some months into HotS since the problems with SC2 in my mind stems from the lack of complexity in the game, sure it might be complicated and hard to be good in SC2 however the game offers no variability on how to be good, which in turn hurts the audience and the "fun" aspect of the game.