Two differences. One, she's using Donald's own language against him, specifically calling him a "loser." Two, she's a liberal woman, as opposed to all the conservative men (and Megyn Kelly) who have been saying these things about him. And if she can goad him into a Twitter war, which seems likely given his complete inability to let ANY insult go without a fight, she can also probably get him to say something misogynistic about a beloved political figure like Warren. No one, even conservative opponents, would ever dare call Warren a "bitch," but with Trump, who knows? And when he fires back with some bombastic nonsense, it's just fuel for the fire for liberals to use come November.