Byron York has an article on Miers on NRO website. Here is the key passage:
The main reason for Miers’s departure is that the White House does not believe she can handle the coming legal fights with the new Democratic majorities in the House and Senate, as committee chairmen demand documents and testimony on the war in Iraq and a broad range of other issues. But the reason many conservatives are glad to see Miers leave is her handling of judicial nominations.
There’s more on nominations, so read the whole thing.