So what makes DFK "deep" and FK not? What's the main difference?

...that it is the amount of tongue involved.  Of course this is a huge YMMV area when it comes to most girls anyway.

It's hot, wet and very erotic with lots of tongue. I like any kind of kissing but DFK is the best turn on for me. You start slow and work your way up to it.
   Kisses Haley

If it's pretty much non-stop action and exchange of tongues for a long or multiple periods of time, then it's DFK; if it's not very prolonged (intermittent) and not a lot of significant tongue action, then it's FK or LFK (light French kissing).

DFK is more intense.  Long pressed soft deep slow tongue thrusts where as FK is lighter and more toned down, in all aspects.