The advantages Warren-D has is she is a generic Democrat
no, the opposite is very much the case.
Exactly. She is an unabashed progressive running against the establishment in a lot of ways, while Brown has doubled down on his reputation as a New England moderate. He won with a lot of support from moderate Dems, and I think we'll see a lot of ticket-splitting in November.
I actually think Warren might have a better chance at beating him with Romney as the nominee, because it's far easier to tie him to someone like Romney than Santorum.