So, I thought I'd put my two cents in on the matter, since I'm pretty passionate about this sort of thing. I'd first like to stress that I have nothing against Markiplier personally. He seems like a very nice guy that really enjoys and respects his fanbase, but I just really dislike videos.
He's very repetitive with the whole "make loud noises, strange sounds, and weird voices" paradigm so many other let's-players do on YouTube.
It's just very annoying when someone reviews a game with little input other than attention grabs. I understand when someone is startled by a game, they tend to jump and scream, but Mark goes overboard with this. I'd be hard-pressed to find a game he plays where he doesn't frantically babble loudly or be shocked with something that is really just mundane.
That, and he really brings little to the let's-play community. The community is already over-saturated with this stuff. The player in a small video feed in the corner of the video, click-bait thumbnails, overreaction to every little detail of a game, an unnatural goodbye message, and not to mention playing a game that has already been played countless times by others. There, the only real reason to watch it is for a new face.
Consequently, his stuff is also very unoriginal, with very little work put in to them.
There is very, very little difference in content quality, and general content in fact, when comparing, say, PewDiePie, Markiplier, Tobuscus, JackSepticEye, and a countless mob of others.
In my opinion, they all seem like they are really just pandering to the lowest common denominator of viewer.
One last thing: I don't blame him for doing this sort of thing, as Let's-players and game reviewers/critics do this stuff for a living. You have to realize and admit that it's just a revenue stream, and they found a niche where they enjoy doing what they do, so in most cases, originality can often fly out the window. They churn these videos out for a living, because it is their living.
Although, there are countless content creators that put time and effort into their stuff, and they also do it for a living most of the time. I might be completely wrong, but for some reason or another, it seems like the ones who put the most effort in their content don't get as much love as the ones who don't. I think this is because the ones who don't keep creating content that drowns out everything else.
Anyways, just a rant, but that's my opinion.