The animation. Let's face it, we remember this show fondly because of the writing, and just ignored how wonky it looked sometimes.

The nonstop speech. Characters, often holding normal conversations, don't seem to know the meaning of the word "pause". When nobody's talking, Peter picks up the slack by internally describing every little thing that runs through his mind, with a special emphasis on the things happening right in front of us. More information than we require, Pete.

The ending. I know the writers expected to return, but it's always frustrating to get a series finale that ends with a cliffhanger.