I found a video comparing two similar clips of old and new AVGN that showcase the difference in acting we've all felt but can't quite put into words.
Two things become very noticeable when you compare these two sketches:
1) His acting has become more stiff and now he defaults to premade faces: the typical angry rant, the angry screaming face, the cuckface when he's surprised.
2) The thick-rimmed glasses hide his eyebrows and make his face less expressive, contributing to the problem of stiff acting.