What is Romney's worst liability at this point in time?
Being Mitt Romney. So I guess "other."
His personal financial secrecy and Bain shennanigans are just part of the larger picture that's being (or about to be) painted. They set the stage and get the attention of the Rust Belt and others who've tangled with Romney's type before.
Ultimately I expect the flow will be:
1. This is who Romney is. A member of the unaccountable financial elite who don't give a shit about you or your family; when their interests don't align with what makes Mittens and his investors money, too bad for them.
2. This is where Romney's been. He's (briefly) been in office before and the results were mediocre. The rationale that one should overlook the negatives associated with point 1 because his ruthless success somehow translates into the public good once he's handed the public trust fails upon scrutiny.
3. This is where he's going. Circle back to point one by pointing at the upwardly redistributive policies he's embraced (e.g. his tax policies, his support of replacing Medicare with a coupon for an Aetna plan, etc). His states policy goals are the same goals he pursued in the private sector: enrich his investors and if the average American has to take one for the team in the process, so be it.
He's an unsympathetic character, and he's starting to run from his primary credential for seeking the presidency. Being Mitt Romney is his liability.