I watch lets plays but I won't just watch any old thing because there's too much shit, here's a rundown of lets plays as far as I understand them:
Speed runs: people with too much free time tearing a video game apart as fast as possible because they know it inside out, normally has no commentary. I once saw someone play through a game PLAYING 2 PLAYERS AT THE SAME TIME! If that's not too much free time I dunno what is. Can be entertaining, but it largely depends on the game and what 'tool assisted' tricks they're using if any.
Serious commentary lets play: The most boring of the 3 in my opinion, unskilled to average players trying to take the game seriously, normally accompanied by uninteresting commentary banter which is usually game trivia.
Casual commentary lets play: the best category I think for entertainment but only if they get it right. Unskilled to average player(s) playing through a game but with some kind of amusing angle to it to make it all funnier to watch, like wacky editing, playing multi-player or even more then one person commentating with good conversation going on. The best of these I think are the ones where the player gets mad over something (but not over do it) or when there's good multi-player banter to cause that rage or other lolz.
of course there's also sub categorys like live recording and prerecorded, prerecorded is better imo. You feel like your time has been wasted less if they do even some half decent editing, not like those fools who think you want to see them play through Zelda 1 on a live feed not collecting any heart powerups.
Then there's blind running and semi blind running a game but those don't really matter as much unless you like watching people fail more.
