The problem is not that he's merely conservative, it is that he is PARTISAN.
http://www.independentjudiciary.com/nominees/nominee.cfm?NomineeID=5
Here's where the partisan bit particularly comes in
Mr. Roberts, a partner at the D.C. law firm Hogan & Hartson, has long-standing and deep connections to the Republican Party. He is a member of the Republican National Lawyers Association and worked as a political appointee in both the Reagan and Bush I administrations. President George H.W. Bush nominated Mr. Roberts to the D.C. Circuit, but he was considered by some on the Senate Judiciary Committee to be too extreme in his views, and his nomination lapsed. He was nominated by President George W. Bush to the same seat in May 2001.
there's more...
As a Deputy Solicitor General, Mr. Roberts co-wrote a Supreme Court brief in Rust v. Sullivan, for the first Bush administration, which argued that the government could prohibit doctors in federally-funded family planning programs from discussing abortions with their patients. The brief not only argued that the regulations were constitutional, notwithstanding the Supreme Court's decision in Roe v. Wade, but it also made the broader argument that Roe v. Wade was wrongly decided - an argument unnecessary to defend the regulation. The Supreme Court sided with the government on the narrower grounds that the regulation was constitutional..
and...
After a Supreme Court decision effectively nullified certain sections of the Voting Rights Act, Roberts was involved in the Reagan administration's effort to prevent Congress from overturning the Supreme Court's action. The Supreme Court had recently decided that certain sections of the Voting Rights Act could only be violated by intentional discrimination and not by laws that had a discriminatory effect, despite a lack of textual basis for this interpretation in the statute. Roberts was part of the effort to legitimize that decision and to stop Congress from overturning it.
The judiciary is supposed to be the one branch that is not subject to political influence, which is why Supreme Court members are not voted on by the general public and are appointed for life. Politics are not supposed to influence their decisions, and removing a need for campaigns was put in place to supposedly remove politics. A guy who has worked for a Republican group of lawyers as well as 2 Republican administrations does not strike me as a non-partisan. He has had political interests in the past as he worked with Republicans and will likely keep his friends in mind as a judge on the Supreme Court.
I am hoping the committee will strike this guy down. If not, he's either gonna be filibustered, or possibly enough Repubs will cross the aisle. I can see some moderates and some hoping to cater to independents for re-election in '06 working with the Dems on this. And the Dems only need 6 of them to get a majority and block the nomination.