HotS is steam-lined like all Blizzard games.
HotS learning curve is less than LoL which is less than Dota 2. Thats all you can really objectively say say about the differences between them all. Which ever you prefer in terms of complexity of gameplay is entirely your opinion.
Nothing wrong with any of them. Its good we have variety in the MOBA marketplace. Personally I like LoL's level of complexity.