A few thoughts:
1. From another beginner's standpoint, JKD is more than the techniques that comprise the "system." It is about the training I think. My instructor often stressed the importance of "aliveness" in our training. I think in a way, JKD can be "applied" to many, but the same techniques can also be NOT JKD. For example, if you walk through a drill such as hubbud, and you're talking about the kids, or what you're going to watch on television, you are NOT doing JKD at the moment. However, if you are focused on your task, and (depending on skill level) try to challenge yourself and your partner, and try to be in the moment, at that point you are doing JKD. Many JKD instructors take aspects from different martial arts, but I think the key is how these aspects are trained. 2. With this in mind, I think JKD doesn't start and stop with what techniques your are doing, but how it is trained. However, I think there is also a focus on practicality and efficiency in JKD. For example, Paul Vunak wrote in one of his books, "JKD is the jab that lands before it is seen." My instructor uses elements of boxing and Thai boxing, but he adapts it for combat. Therefore, some of the techniques, such as the Thai switch round kick, we use but we shorten the switch as much as possible for the most power. Also, we modify our boxing covers to account for the fact that we don't have a boxing glove on, etc.
3. JKD is about the process as well as the system. When you are training, I think it is that you are doing JKD as opposed to knowing JKD. Bruce Lee spoke a lot about self-expression. To fully express oneself, one must be fully in the moment. It was humorous one day when I was sparring with my instructor. I was trying too hard to get in a certain technique, or to do a certain stop kick which I was waiting for a certain entry. After the round, my instructor told me to "Just be. Let it come out." After that, I was surprised as the same technique I was trying to do came out much more easily as well as several others, and I wasn't able to pull of a stop kick, but other defenses/counterattacks I had trained more heavily were coming out more successfully. I was--to some extent--just "being" and therefore I was more relaxed and let my training come out. My mind was more alert as I was not "looking" for a specific thing.
Anyways, just some thoughts on what I would say...by the time I was done, I'm sure the person asking would be done listening!