For the past few months, I've been trying to wrap my head around this whole X3D thing. I haven't really done anything with it yet, but I have read a lot of documentation, and it just isn't clicking with me.
I think the problem is that when most people decide to learn X3D, what they're looking for is a nice universal file format for 3d models. My impression is that to work with 3d models now, you have to go about downloading various SDKs describing how to work with certain propriety model formats that are outputted by programs that you could never afford. Developers want some sort of standard file format that they can work with instead, and ideally X3D would be this standard. What they actually get is VRML encoded in XML.
Now VRML sort of puttered out a few years back, and I suspect that this would probably explain the lukewarm reception of X3D right now. Most people that I've talked to have either never heard of X3D or simply don't care.
In any case, because of X3D's VRML past, it actually does three things:
X3D describes a way of encoding 3d geometry in XML. This part of the specification is what people actually want X3D to do.
I think X3D also defines an API for storing and manipulating scene graphs in memory. In fact, the file format is simply a way of serializing the scene graph to a file. I'm a little ambivalent about this. Admittedly, every time anyone wants to do anything in graphics, they have to put together some sort of scene graph code. Each year, thousands of computer science students taking graphics course have to put together a new scene graph framework so that they can do some 3d work. And anytime a company decides to create a 3d API (e.g. Java3d or JSR184), they always decide to create their own scene graph API to go with it. So I suppose it would be nice if everyone could agree on a single scene graph API. On the other hand, the first thing that people ask upon encountering a scene graph API is "how do I make it show a bunch of triangles on the screen like in OpenGL," so maybe a standardized scene graph API isn't all that useful an abstraction. After all, you don't see people creating a bunch of objects for representing shapes and geometry of 2d scenes. I guess the thing that causes me the greatest concern is the fact that all the X3D implementations that I've looked at (which is, admittedly, only two) take the X3D scene graph and immediately convert it to a different scene graph format before doing anything with it. This includes the X3D reference implementation! If the scene graph API were any good, then there really shouldn't be any reason to convert it to a different representation, right?
Because of its VRML roots, X3D similarly tries to provide a complete "platform" for doing 3d applications. In other words, X3D seems to provide enough language bindings, scripting support, interactivity features, event models, etc. that X3D can be used as the basis for a complete 3d application. Personally, I think this is the most dubious aspect of X3D because understanding and implementing all of these features is really hard, and people who are only interested in a universal 3d file format really don't care about this stuff. I think it may be possible to make a compliant X3D implementation that ignores all of the platform stuff, but I'm not entirely sure yet, and in any case, I find the X3D documentation extremely hard to read because of all the references to these platform features.
So I guess the primary question is whether X3D fails as a file format because it tries to do many other things as well. Frankly, I don't know. I have committed myself to doing some work with X3D though, so hopefully I'll be able to figure this out as I dig deeper into the specification.