Several quick thoughts:
1. Although Ms. Kagan has not served as a judge (like Ms. Miers, though Ms. Miers also had never argued a case before the Supreme Court and had written very little on the law), it remains to be seen whether Ms. Kagan will suffer from the same kind of inability to address questions/provide answers that plagued Ms. Miers during the "coaching" sessions. In other words, even as she lacks judicial experience, does she possess additional strengths that would allow her to overcome that lack of experience?
Concern about Ms. Miers' capability to serve on the Supreme Court was shared by liberals and conservatives alike. For example, following her withdrawal from the process, Senator Trent Lott
explained, "I just was concerned that she was not strong enough, dynamic enough and had enough experience in the constitutional area to be on the Supreme Court." Some other prominent conservatives e.g., Judge Bork, were much harsher.
2. Ms. Kagan, if she is appointed, would be replacing a liberal justice. The overall tilt of the Supreme Court would not likely be impacted by her appointment, unless she proves more conservative than expected (sometimes Supreme Court justices turn out differently from what had been expected).
3. Even without judicial experience, Ms. Kagan has legal experience, including experience in the area of constitutional law, (academically at University of Chicago and Harvard Law School and also as Solicitor General and previously in private practice).
My initial guess is that some Senators may well seek to delve into an early
thesis she had done on the socialist movement in New York City. But ultimately, if she can express her thoughts in a coherent fashion and demonstrate her knowledge of constitutional law, she will be confirmed, especially as her nomination does not alter the Supreme Court's balance so to speak.