Two myths I frequently encounter in blogland are (1) that the policy positions of Obama and Clinton are not that different and that (2) Obama and Clinton are about equal in terms of their electability.
Hilzoy, Jonathan Chait, and the Anonymous Liberal do a pretty fair job of challenging the second position.
As to the first point, even my sparse attention to policy details during this campaign reveals some rather startling distinctions between them.
There's a flip side to this of course, as one could point out Obama's allowance of anti-gay ministers to officiate at his events, his favorable remarks regarding Ronald Reagan that sent much of the left blogosphere into a tizzy, his use of the term "crisis" to describe Social Security's financing, etc. So the slate isn't completely clean on Obama's side.
But Obama's stances seem to me to represent the biggest break with the past as well as the present state of policy discussions, both foreign and domestic.