For the complexity topic: I understand very little about, say, League of Legends, but I understand that the main goal is to fight your way to the other end and blow everything up, and preventing the other guy from doing the same. And, as I discovered when going to Melbourne for PAX, I also understand very little about, say, AFL, but I understand the main goal is to get the ball to the other end of the field, and kick it between the posts.
Watching either, I'm mostly lost as to the actual tactics involved, and a lot of the rules... why do they fight over the ball so much after a tackle, if the umpire's just going to take it back and throw it in the air anyway? In what ways is one hero better or worse than any another, how do they all interact? And listening to conversations between fans, I have absolutely no idea what's going on... ("So then Collingwood was playing the Magpies at the Telstroptus Oval, and you know what they're like playing away...", "And then they tried to use Teemo as a tank in the top lane, can you believe it?"), it all sounds like "Football football football, football football. League league legends league, LoL. Derp derp, I'm Alex, this is how I talk."
But, I understand enough to get some amount of enjoyment out of watching a game, and while I can't exactly follow who's ahead in the middle of a skirmish, and the commentators don't usually help much (being aimed at someone with more knowledge than I), when the dust settles and points have been scored, I can get some enjoyment out of the big-picture game. And I picked up some amount of the details from watching... like levelling up in LoL, and apparently there's some kind of store (but no idea what's in it), or that catching a kick on the full is a thing in AFL that gets you the option of a free kick (but no idea what decisions go into whether you take it or not).
So really, I don't think complexity is a major factor here - both physical sports and esports have a lot of complexity if you want to go deep, but the good ones you can still follow without going that deep into it. And, on that note, as mentioned, there's Cricket