Agree that Dem voters could help Trump, but not by voting for him in the general. There is plenty of disdain for Hillary on the left, no question. There's also a ton of contempt for Trump by these same folks. I think they help Trump by not turning out to vote. Some will vote for him, but his biggest benefit will be them staying home.
I am not an expert on her detailed politics so I'll answer this in a very general manner.
1) She is intelligent and communicates well. We all know about Dershowitz commenting on the intelligence of Cruz, but he's also spoken about Warren's. He's made statements singling out Cruz and Warren as two of the smartest members of the Senate.
2) She has charisma. I was astounded at the level of support she generated with those on the left. The Warren support spread like wildfire, and her ability to connect with people and fire them up is a key component of her allure.
3) A sort of political Goldilocks effect. Just as we see many GOP pushing for candidates further right, Warren's ideology seems to be in the left's 'sweet spot' - not as extreme left as Sanders but left of Clinton. Seems to be a popular spot with many on the left today.