...and while DFK is definitely part of GFE, the emphasis should be on the overall experience and chemistry created. Also, my definition of DFK may be very different from yours. DFK isn't just about tongues flitting about. It's about sensuality, bodies and mouths embracing, and the chemistry we have together.
Tabu summed it up quite well, I might add.
Here's my personal experience concerning kissing:
Older gentlemen have a tendency to have very, very hard tongues. I personally do not enjoy a hard tongue in my mouth, and will very quickly correct someone who has this issue.
I'm also a person who concentrates a lot on the mouth and lips; nibbles, sucking, etc. The tongue action should move seamlessly into everything else, and not be its own entity. Making out/kissing/deep kissing gets me REALLY excited when done correctly.
I kiss the way I like to be kissed. End of story. If you're not sure what you're doing, pay attention to what I'm doing. If you like to be kissed a certain way and I'm not doing it, please tell me! While I'm excellent at reading body language, I'm not a psychic.
As a GFE provider, DFK is always on the table, though the intensity will vary from person to person depending on chemistry. I've had some clients who weren't into kissing at all, and I found the experience strange and uneasy.
Anyway, hope you find what you're looking for!