I like Warren a lot, but she is not a viable candidate. She barely won Massachusetts. We need somebody who can win Ohio, Virginia, and Florida. That's not her. A primary vote for Elizabeth Warren is a vote for a Republican President.
Look, we can discuss how blue Massachusetts is all you want, but Brown still was a very popular, strong incumbent. Granted, he was elected by a very special set of circumstances and him surviving a regular election, with full presidential turnout, was always problematic, but it was not an easy race and not because Warren was a "weak" candidate. I mean, we always knew this is going to be a close race.
Does the election in 2010 make her a strong presidential candidate per se? No. But she's definitively not a lightweight.