My primary problem with Bernie is that imo, he is an absolutist. I was once a big fan of his but once I really took a hard look at his record, I was very disappointed to realize that he rarely if ever seems willing to compromise. For example, he refused to vote for the auto bailout, despite supporting it, because it included the bank bailout. Most liberal economists explained that the bank bailout was necessary in order to prevent a much more destructive economic crisis. I'd call the bailout a necessary evil. Bernie has a history of sticking to his principles even when compromise would be a much better option. It's fine to have principles and ideals, but the only way effective government works is when both parties comes to terms with the fact that "you can't always get what you want." In other words, legislation usually has to happen somewhere in the middle. I've had enough of obstructionist Republicans. I sure don't want to see the Democrats go in that same direction.
Secondly, Bernie doesn't seem to understand just how difficult it would be to implement any of his unrealistic ideas. As a health care provider with over forty years experience, I can tell you that the US is in no way ready or capable of having single payor care for all. We have enough problems funding Medicare and there are many areas that need to be changed. How in the world does Bernie expect things to change? Does he honestly believe he can get the cooperation of the other side for single payor? If so, I'd say he's delusional. If not, he's just being another politician who makes empty promises that he knows he can't make become a reality. He speaks constantly of a revolution. Yeah. Right. We sort of tried that in the 60s but it didn't turn out all that well, despite all of our efforts.
Finally, I haven't heard him say much about foreign policy. That worries me. But, if you think Krugman, who I highly respect, is being hard on Bernie, you should hear what Barney Frank says about Bernie. It's not good.
Regardless, I will vote for Bernie if necessary because despite all of the things about him that I don't like or worry about, the Republican candidates scare me a lot more. It's foolish, imo, to think that you should only vote for someone you completely support. Nobody will ever meet all of my ideals so all I can do is look at the choices and decide who has the chance of heading the country in a better direction than the others.