Pewdiepie is a horrible, horrible LPer for three reasons:
1. He is afraid of silence. The guy won't shut up for four seconds, and he runs out of material far too quickly because of it.
2. His 'material' consists of:
a. conversing with on-screen characters and objects in silly voices
b. use of highly overdone internet memes ("u mad bro?", "FUUUUUUUUUUUU", etc.)
c. narrations of literally everything he is doing onscreen
d. excessive fanservice (BARRELS! Piggeh. CHAIRS! and let's not forget Stephano)
e. unnecessary screaming
3. Picture-in-picture Let's Plays. If there's one thing I hate to see in a Let's Play, it's the player's face in the corner. You can see him forcing himself to overreact, and you can watch him as he tries to be 'cute.' His awareness of the viewer's ability to see his every move pushes him to feel the need to 'preform,' and everything ends up being forced.
Goddamn kids wouldn't know a good LPer if it bit you in the ass.