I'm a romantic kisser to start with, I love liplocking, gently bitting and sucking lips. Kissing is the art of making love with your lips. I love french kissing, and I believe tongue is something we add to make kissing even more intimate and erotic.
Sensual kisses turn me on a alot more than agressive kissing that are too fast and too deep. I don't like having a tongue shoved inside my mouth, I feel violated. And when a kiss is only tongue, with not even a bit of liplock, then that can't be called a kiss nor a french kiss... it's just tonguing.
So for me... Kissing is when we liplock only, with no tongue... French kissing is when we liplock with tongue, and it can be either LFK or DFK... LFK is just regular french kissing, a mix of liplock with a bit or a lot of tongue action... DFK is when the "D" excercises its full meaning; literally shoving your tongue deep inside your partner's mouth, licking his/her teeth, molar and glands (yes, some guys go really deep)...
That being said, I am not a DFK person... I don't think tongue's should be inserted in one's mouth in order to french kiss. Both persons should stick their tongues outwards just enough to touch the other when they liplock. Again, my mouth feels violated when tongues are shoved inside, deep enough that I could bite it. And I hate it even more when a tongue is too stiff...