My issue with her isn't so much her ideology. I think she's probably a conservative (although a moderate and opportunistic one) at heart. My issue is with the fact that she:
1. Has no particular qualifications aside from being a personal friend of Bush. Neither experience nor exemplary performance/intellectual weight is in her record. As a matter of fact, commentators have noted her complete lack of any particular evidence of ideological courage (which is absolutely necessary on the Supreme Court if one is to not move left like Souter, O'connor, or Kennedy)
2. Is over 60 years old. This is not a disqualifier in and of itself, but it does tell me that Bush clearly was not trying to make the choice that would have the biggest long-term impact on the Supreme Court, and I consider it a wasted opportunitiy to choose as a replacement to O'connor a justice only 14 years younger than her... particularly when there were far more qualified justices over 10 years younger than Miers.
You hit the proverbial nail on the head.
I trust the President that Ms. Miers is conservative, but that is not enough, by itself, to qualify one for the USSC.
(1) As you point out, she is too old. I want a "young" conservative. 50 at the outside. Preferably in their mid-forties, who will be on the court for 30 years.
(2) I want an intellecutal powerhouse - a Judge Bork (now, sadly too old), Judge Luttig or the like. Someone who, like Justice Scalia, can cut through the convoluted and ugly logic of the liberal on the court.