The US Constitution provides that the president nominates Article III judges subject to the advice and consent of the Senate. As I understand it traditionally, this advice and consent role was viewed as a way to guard against the appointment of obviously unqualified people. As this list on the Senate's web site shows, plenty of nominees for SCOTUS have been opposed vigorously through the years or even defeated. But at least for the last 100 years or more the great majority of Supreme Court nominees have been approved, many by voice vote (can anyone imagine such a thing today?). The logical conclusion is that most nominees were qualified and were approved based on that fact. In effect elections mattered and the man who won the presidency got to decide who sat on the court.
Shortly after George W. Bush took office, the senior senator from New York, Charles Schumer, decided that ideology should play an important part in the judicial selection process and his office put out a press release trumpeting this view. As Schumer reminded us during the Roberts hearings this started as a minority opinion among the Democratic caucus but he soon convinced the majority of his colleagues from the left. This led to filibustering of appellate court nominees, ended (for now) by the Gang of 14. The Democrats now feel justified in opposing any nominee with a paper trail supporting conservative opinions simply because they do not think he will produce the results that they want. There is little expectation that a highly qualified but outspoken nominee in the mold of Justice Ginsburg could get the 90+ votes that she received.
So where does this leave us? The president made a campaign promise to nominate justices in the mold of Scalia or Thomas. But even with Republicans holding a solid majority in the Senate he can't do nominate someone with that kind of record because either the Democrats would filibuster or the Republican caucus would be likely fracture over the abortion issue (Snowe, Collins, Chaffee and Specter would be likely to oppose an avowed anti-Roe nominee). In fact any nominee with a clear conservative record will be very difficult to confirm. This tends to eliminate many of the brightest lights of the judiciary or academia. Judges like McConnell, Luttig or Posner who are eminently qualified and have written extensively would ignite a firestorm. As much as the base on both sides would love to see a knock down, drag out fight, politicians by nature don't like to go this route. It provides too much risk, and risk isn't conducive to maintaining political power. (All politicians tend to be conservative in this respect.)
Instead we get a nominee with lesser credentials and no paper trail. In essence the president trades qualifications for confirmability. The imposition of an ideological test results in justices who are less qualified and country loses the possibility of service from some of the best and brightest.
This is not to say that Miers would not be a good Supreme Court justice. She might be. However, there is little reason to think that she is the best person for the job. I would rather have the best legal minds on the highest court in the land. But I suspect we will face the prospect of the best being out of contention unless and until one party achieves a stable, filibuster proof majority.