Here's the problem with the 2016 Hilary Clinton candidacy: she is deeply unlikable in many circles. That, and it seemed like on the 08 trail, the more we saw of her, the less we like her.
What circles would those be? Among Iowa Democrats, opposition to her is practically non-existent:
http://www.publicpolicypolling.com/pdf/2011/PPP_Release_IA_072312.pdf
And a recent Florida poll of likely voters shows her favorability with registered voters, Ds, Rs, and Is, is a strong 68/26%: http://www.suffolk.edu/images/content/FINAL_WED_FL_Marginals_May_9_2012.pdf
Only problem is, I think she's serious about being done with elected office. And even if she did run, a presidential campaign would wear those lofty numbers down. But make no mistake about it, if she somehow did choose to run, she'd be formidable not only against a President Romney, but possibly even in the event of a re-elected Obama.
i have no qualms about her being formidable, but again, with the right circumstances, and some good opponents, which I certainly see in Schweitzer and Cuomo, she could be quite diminished, and possibly even lose again. The democrats seem to dislike establishment.