He was such a huge and dominating character in the show (and I would argue one of the 5 best TV characters of all time) that once he left they had to magnify all of the other characters to make up for it. What was great about the office is there were no stars in the original cast and every character felt like a normal person that you would see in a paper supply company office. Once Michael Scott left and they had to fill his void by magnifying every other character on the show, they suddenly became less believable and that original feel that the show had was pretty much gone. It was still good for some laughs every once in a while but it was an entirely different show.