The conservative/ reactionary blogosphere has adopted the tern Overton Window to express the range of 'acceptable' (i.e. non-crazy, non-evil) political opinion enforced by the mass media; along with the idea that this window moves ever-Leftward.
This is a useful shorthand in some ways, but misleading in others. The Left is not fundamentally about a different set of opinions (through its history, the Left has picked-up and discarded 'causes' with extraordinary and increasing rapidity); the Left is about destruction of The Good - it is about subversion, inversion of truth, beauty and virtue; propaganda and enforcement of untruth, ugliness and vice.
Insofar as there is a 'window' - it is therefore defining the scope for acceptable attack on The Good.
Insofar as the window changes, it does not really slide to the Left, but enlarges.
So the change Leftwards should be seen as a greater and greater scope for 'acceptably' critiqueing, problematizing, and in multiple ways attacking The Good.
The window has enlarged recently to allow attacks on various new themes - attack on marriage via the recognition and encouragement of non-marriage; attacks on the family by promotion of everything-but; attacks on sanity by the legal enforcement of individual person's delusions; attacks on health by the encouragement of behavioural/ psychiatric/ sexual diseases (disease being here biologically defined as a condition which damages probable functionality, lifespan or reproductive success).
And so on.
In sum - if we use the Overton Window metaphor - we should regard the 'window' as enlarging the scope for 'acceptable' attacks on The Good and the 'acceptable' scope for active promotion of evil.
Note added: Consider the 1960s sexual revolution. This reveals clearly the expanding range of behaviour it was acceptable to first accept, then approve, then advocate and finally (recently) actively promote and enforce. Divorce, sex outside marriage, promiscuous sex, non-reproductive and anti-reproductive behaviours and orientations. Indeed, most of the heavy-lifting against Good was done in the late sixties and early seventies with legal decriminalizations and elimination of negative moral status in the mass media. Recent expansion of the Overton window have been mostly about the escalation from illegal to officially-promoted; not-disapproval to approval, not-forbidden to actively-encouraged.