Primarily her foreign policy. Hillary is a known hawk and her consistent criticism of Obama is that he hasn't been militarily aggressive enough.
Plus every time she does use our military, she always screws it up: see Lybia & Syria coupled with her bizarre admiration for Henry Kissenger, one of the worst mass-murderers of the 20th century. The woman's going to get us into another war.
Also she's nakedly in the pocket of Wallstreet. I mean Obama is too, but not to the brazen extent that she is, and her and her husband are true believers in neoliberal deregulation which originally opened the door for the 2008 crash - so a HRC administration is not likely to do anything substantial to prevent a second crash, which makes another financial crisis like 2008 infinitely more likely.
Finally, I have hated Hillary Clinton my entire life - sometimes for bad reasons and sometimes for good, but seriously she has always rubbed me the wrong way. I don't believe for a second that she cares about anyone but herself and her career.
I don't want our own American version of Margaret Thatcher, the world already had one Margaret Thatcher and we're still recovering from her.