Guess I'll start. I'm actually rooting for Sanders, and would vote for him over Trump or Clinton, but no one is perfect.
Actual problems with Sanders:
International policy is not his strong suit, when the US are in their position of controlling so much of the world's security, their President should have a strong sense of foreign policy. Sanders is far weaker than Trump or Clinton in this, experience wise.
His economic plans attack the 1%, the people that control the country. How is he going to pass any laws when the government plays "the game"? Spoiler: It's gunna be a big struggle. This ain't Vermont anymore, this is the big leagues, a whole new ball game, and one I don't feel Bernie understands as much as the other candidates. Lots of growing pains for an old man. He will need to fight hard to get even one of his plans to come 1/2 way in 4 years.
If Sander's plans do come to fruition, where does the money come from? Free college and universal healthcare are very significant proposals with a heavy price tag. Putting more control into the gov't isn't every American's best interest. And with the previous point, it ain't just gunna come from the 1%, sorry.
These were just off the top of my head, I don't consider myself the Abraham Lincoln of politics (anything but), but it shouldn't be too hard to figure out some side effects to the Sanders medication.