I suppose the two main concerns about her moving to State have already been made by others. The first, as Jon Stewart noted, is that the only area of substantive disagreement between Clinton and Obama has been foreign policy. Seems like Obama's just asking for trouble. The other concern was capably expressed by Josh Marshall:
I think we should consider that during her time on the national stage Sen. Clinton has been at the helm in two big undertakings -- had two big executive leadership tasks. One was health care in 1994 and the other was her presidential bid in 2007-08. Each was something of a trainwreck from an executive-level management perspective. And the State Department is a notoriously intractable bureaucracy.What's so bad about the Senate?