Mark Thoma thinks that pundits on the left are more willing to criticize their own team than pundits on the right. My guess is that people on the right think it's the other way around. If so, then let me propose an explanation.
I think that when pundits criticize someone from their own team, it is usually complaining that the politician is too moderate or too ineffectual. That is, leftwing pundits criticize President Obama either for not following policies that are further to the left or for not championing left-oriented policies more strongly. Similarly, pundits on the right criticized President Bush for not following policies that were further to the right or for doing a poor job of articulating right-wing philosophy.
These sorts of criticisms are difficult for the other side to notice. It may be hard for someone on the right to think it is a big deal when a liberal politician is being criticized from the left. And those on the left do not always notice when a conservative politician is being criticized from the right. I mean, you may notice such a criticism, but you don't "count it" as a criticism, because it's the opposite of the criticism that you would make from the other side.
Of course, what would really be interesting would be a pundit on the left criticizing a liberal politician's liberal policies, or a pundit on the right criticizing a conservative politician's conservative policies. I believe those instances are relatively rare. One example might be the Washington Post editorial page, which has criticized the domination of the teachers' unions in Maryland politics. Another example would be conservative pundits who have criticized Grover Norquist for his strict stance against any tax increases.