The stat-tracking, especially your rating, which is dependent on win/loss rates and therefore on how good your teammates are.
The fact that there's a "pro" scene, which far too many people see themselves as part of.
The amped-up "anonymity" part of the equation, since (especially in public matchmaking games) you'll play with dozens of complete strangers every day, rarely if ever seeing any of them again (think dungeon-finder World of Warcraft vs traditional chat-based LFG World of Warcraft).
The fast-paced, intense, competitive nature of the game. Playing any MOBA game is pretty much the opposite of chilling out.
Regardless of how complex it may be tactically, most things in-game, from choosing your character onwards, have an optimal way of being done, which someone on the Internet has written about, and which many people will automatically expect you to have read about and memorised.