And ultimately, there was all the "Huh?" when ubervamps were dispatched like so many lemmings.
In hindsight, I assume that the First Evil was supercharging its initial übervamp just as it did with Caleb to make it more than a match for Buffy. The line that it was in "remission" after its plans via the übervamp failed would tend to lend credence to this, as I can't quite see the grief from its death alone being enough to keep the First from pressing the fight.
Also, the Slayers had a very sharp magically unbluntable weapon with which to cut off the vamps' heads in the finale, and presumably their experience playing keepaway with the last bag of oreos while crammed 20 to a room in the Summers house helped immensely with passing it effectively from wielder to wielder.