Actually it depends on your perspective. Many people actually think that CNN is somewhat conservative. Think Glenn Beck, they give him two hours during prime time to rant about whatever he thinks the country should know. Personally I perceive Glenn Beck as radically right.
If you were to compare CNN with BBC, or even CNN International in terms of international news, you can still see that it does have a conservative bias to some degree.
As far as the debates go, asking the candidates to clarify what they said about each other is simply a waste. In that way, CNN is simply trying to create some sort of reality TV show with these elections, getting one politician to attack another all in the name of higher viewership. Maybe I'm wrong, but I can't think of any other reason Blitzer would ask Obama to comment on Clinton's remark made two weeks ago about some minute detail of no national importance. I honestly think by asking harder questions, they are giving the politicians some respect, allowing them to convey their message to all the viewers.