The semantic difference lies in whether one has doubts or qualifications on ones opinions, at the very least. Equating "I disagree" with "is wrong" presupposes that you are not mistaken and cannot possibly be mistaken, therefore if you disagree with something the problem must be with the something, not with your opinion.
Along similar lines, consider the difference between "I don't want to do X" (or even "I won't do X") and "I don't think anyone should do X".
Or the difference between "I don't think anyone should do X" and "X should be illegal".
From my point of view, all three situations are analogous and the difference in each case is not just a matter of tone.
