I play HotS most days. I just find it to be a blast.
I wouldn't necessarily call it simplistic, but I would certainly call it streamlined. I just figure that it really trims the fat off of a lot of other MOBAs, and it has a deadset focus on teamfights, and objective play. Thats what MOBA's are about, right?
I came from League, and I had a lot of fun playing it, but the games were too long and bloated, and not to mention that I just had a fundamental disagreement with runes and masteries, which can give players advantages before the game even begins, and they discourage the player from dabbling in other champions and roles.
I mean, each to their own, in the end. I have never had a problem finding games in HotS, so even if it is the least popular "big" MOBA, the game still has a big enough playerbase to ensure that the game is still going to be around for a very long time.