Different reasons... It can be hard to put your finger on. He seems like a good guy, probably a great husband and father...but at times it's hard to see what exactly he brings to the table, or what he stands for. His podcast is usually about having discussions on a wide range of topics. Open dialogue. It can be thought provoking at times, but the dropping of multiple f bombs when talking about something like books on nature and conservation is head scratching and (to me) unnecessarily crude.
Rinella has strong beliefs and opinions that he stands for, while still having an open mind and asking questions, O'Brien acts like a river. Winding and flowing down the path of least resistance, while never giving listeners anything solid. Take a stand for Gods sake man, ask the hard questions!
But to be fair, I have enjoyed some of his podcasts and guests. Others I can't stand.