Warren's stand against the banks criminal behavior is what brought her to national prominence, so I get that she is what keeps people like Bloomberg awake at night. But beyond that, Warren is attempting to woo the Hillary Clinton/establishment vote by retreating from M4A and running to Bernie's right. Her foreign policy, as best I understand it, is no different than what we've seen for the last 40 years.
So for Bloomberg to view her as a big threat to his way of life tells me he is what we thought he was... a republican elitist. The fact that he is able to run as a democrat without any serious push back from the party's leadership demonstrates the corrupting nature of money, especially those with a Billion or more to spend on political propaganda.