What I mean is that they introduced to many characters to fast, and they were unneeded. Erin for example. Once she stopped dating Andy she became irrelevant, yet they kept her around.
Shortest way to explain it is: The writers introduced too many characters (Erin, Gabe, the fat kid) , and created unnecessary plot lines when they already had characters to work with (Base crew: Dwight, Jim , Pam, etc etc)
The Stamford merger was a proper example of how to introduce characters. They started with five new people, but narrowed it down to two characters that had established personalities and worked with the flow of the office.
Meanwhile, in the later seasons, they just throw characters in there just to add unnecessary bullshit that could be avoided.