The Taliban don't want a protracted civil war in Afghanistan. This seems difficult to comprehend, given the last 30 years or so of fighting.
But it should be made clear right away--fighting an invading power is much different from fighting a full-on internal civil war. Defeating an occupying power requires great patience, but the Taliban can largely avoid losing major resources and manpower by avoiding a NATO army that is not large enough to cover the entire country.
Fighting a civil war is much different. No waiting will do--fighting against the central government and its own Warlord allies will be a knock-down drag-out war of attrition, and one the Taliban may well not win, if US intelligence and drone attacks remain on Kabul's side. Such a civil war would be devastating, even if the Taliban won.
This scenario is very unlike the complete power vacuum left when the Soviet Union withdrew--the Soviets had not conducted in massive, extensive state-building that NATO has. The central government may not have driven the Taliban out of much of the country, but it does not mean the Taliban is close to secure.
The other point worth keeping in mind right now is that a player in a war or other disagreement wants most to negotiate when its relative advantage is peaking--that is, the point that it's going to be able to squeeze the most out of the other side. After peaking, time erodes the advantage, and less can be won.
Thus, the Taliban are coming to negotiate. Not the big-wigs (like Mullah Omar), but some of the elite near the middle that are feeling the pinch and the pain. Omar himself can't afford to appear to be willing to negotiate with an occupying power for an instant--too large a proportion of his followers (as well as is Pakistani ISI patrons) oppose such negotiations, and are likely to outright kill him if he becomes a collaborator.
The same risk applies to the leaders coming to negotiate. That said, they are not nearly as safe as they used to be in Pakistan, given the uptick in the frequency and success of US drone attacks against leadership of this level.
NATO is doing an excellent job of promising and reliably giving safe transport and harbor to those coming to negotiate--it might be personally safer for many of these leaders to come to talk, rather than hide in Pakistan.
But, ultimately, the top Taliban leadership is not declaring any readiness to negotiate--even if they may be leaning towards it. But in the end, the Taliban will not be able to sweep through Afghanistan and claim victory as NATO leaves. There will either be a negotiated settlement or a crippling civil war, and the former is more likely.
The biggest deciding factor for the top Taliban leadership will be whether they actually think their advantage is going to grow into the future. If it becomes obvious that their bargaining position is declining, the hard line against negotiations from the top will begin to soften--and many of the middle-level elite leaders will already have the initiative in talks.