A very helpful post.
"instead try to identify and agree on what core elements made TFC fun for different people"
Boom. This is it right here.
What makes FF fun for me is the focus on movement abilities vs fragging abilities.
In a CTF environment, (and many AvD environments) the two teams have wildly different focuses.
I love having movement techniques to get flags, and I enjoy having tools to stop those movement techniques when on defense.
Concs offer movement of yourself and movement of the enemy.
Projectile weapons offer momentum to stop attackers.
This is why I'm so gungho about a grappling hook for the spy. Right now spy has no special movement techniques and he's ostensibly an "offense" class.
Every offense class should have some sort of movement technique.
The beta scout movement gun, the jpad, concs, whatever.