I have been struggling with RDF myself, because of my peripheral involvement in the Chandler project which uses RDF extensively. And I am beginning to think the real problem is that RDF is too simple...
How do I explain this? Well, RDF is made up of only a few rules. You have statements consisting of a Subject, a Predicate and an Object, along the lines of "Charlie is the father of George." You can make multiple statements that connect via Subject or Object Nodes, so I can add that "George owns the Hotdog Hut." "The Hotdog Hut is in competition with the Burger Palace." "The Burger Palace is managed by Charlie."
There, four statements that encapsulate an awful lot of information about Charlie, George and their relationships. Add to this that I can tack on other information to each of those statements, like for example the menus of the Hotdog Hut and the Burger Palace. And I can group statements such that I can say "Charlie is the father of George, Martha and Peter." What is more, I can make 'statements about statements' like "Jack said all of these things."
Still with me? OK, from a purely lexical point of view RDF is simply a formalization of normal English sentance construction rules. In other words it is a way of describing things that has much of the power of the English language. Thought about in those terms you can begin to grasp the range of things you can do with RDF, but you will never be able to apply it!
However, in a mathematical sense what we are referring to here are Nodes and Edges in a Network Graph. In this case 'Nodes' are points of reference (for RDF they are URIs like 'http://www.google.com/') and 'Edges' are the predicates that define how Nodes are related. All this is done with a simple syntax that can be (but does not have to be) represented in XML. Anyone who has some grasp of the numerical theory behind network graphs can apply RDF, but even they are going to have some difficulty conceptualizing it.
Why is this? Didn't I start out by saying it was simple? Well, you see the problem is that very complex scenarios can emerge out of the interaction of a few simple rules. This concept of Emergent Behavior is a hot topic right now because it promises to finally explain (among other things) how our very minds can operate in a meat matrix of simple cells with simple connections. The trick is that you need lots of those simple cells.
RDF is an emergent system consisting of a network graph realization framework made out of a few simple rules. The interaction of those rules open up an entire universe of possibilities and it is difficult to get your head around an entire universe. Of course, so long as you are only dealing with a few things (Charlie and his son George) you are OK. But the minute you bring in the rest of the town things get ugly fast. RDF is designed to let you model the relationships of the entire town. That is why RDF is hard.