I'd put Let's Plays somewhere above reaction videos. In a Let's Play the subject is working with an active medium and their personal performance has an effect, whereas a reaction video it's just a passive medium being reacted to.Probably for th same reason why my favorite "Let's Play" videos are blind playthroughs. It's more fun to see somebody's first reaction to something instead of seeing gem play something they already know about. It's why I never watch any of Michael Damiani's playthroughs because he's usually playing something he already knows inside and out.
But many times, I am watching a Let's Play to see how they react to the story within the game.I'd put Let's Plays somewhere above reaction videos. In a Let's Play the subject is working with an active medium and their personal performance has an effect, whereas a reaction video it's just a passive medium being reacted to.