I agree with Boomer & Greaper that the Afghans shouldn't be underestimated. They outlasted the British and the Russians. Other thing is, unless we want to slaughter everyone in Afghanistan, we run the risk of screwing ourselves into a long-term engagement, which might escalate into a regional conflict.
Thing is, rather than direct involvement, we should support the anti-Taliban forces that control the last 5% of the country. I don't think that the Taliban can be all that popular, considering that they rule with force.
While I think the Afghans are tough, I also think that if we go after the Taliban, they will look like they had been worked over with a rake. We would bloody massacre them.
I don't think North Vietnam would have held out if we fought no holds barred and NVA hadn't had the Soviets and China helping.
Iraq had a combat-tested large army, and look what happened to them.
Bring it.