Warren has been coddled by the media so far and hasn't been confronted much by the other candidates. She's shown in private interviews that she's not as solid and articulate when she's confronted. She may not stand up well to serious scrutiny, and now that she's becoming the front-runner she'll have to. If the Dems are smart, they'll keep Tulsi Gabbard off the same debate stage as Warren, or it's only a matter of time before Tulsi attacks Warren on foreign policy.
Personality-wise, Warren comes off as a slightly zany, even flighty soccer mom. She doesn't "look the part" of a strong president to me, but that could be partly my own biases. Some of her positions and proposed social programs are impractical to the point that she sometimes looks out of touch with reality. She may be more electable than Bernie, but that's debatable.
Hillary Clinton was cold, ruthless and elitist, but from a practical standpoint, she could have held down the job of president. I'm not seeing that with Warren yet. Maybe she'll step up.