It will be difficult for Madrid to win Juve another time, but they have done that with Atlético. Another small advantage is that the second leg is at the Bernabéu. So it's not out of question, but it's far from sure.
As a matter of fact, people give little chance to Roma or Sevilla, but they could obviously pull out a surprise. I mean, who would bet Roma was to leave Atleti out of the Champions or that Sevilla would eliminate Moi's United.
Basically, at this stage there are no weak teams. It's just few games anyone could win.
That being said, Barça is showing outlandish consistency. The only way they lose this Champions is a sudden dip in form.