In Peter Unger's Beyond Inanity, he makes a point of saying that the claims he's attacking are guilty of being insubstantial, not silly. However, it strikes me that an awful lot of what gets done in philosophy is silly. I've even suggested that philosophy is so silly, that an obviously silly paper is actually an improvement over one that tries to hide its silliness.
What, exactly, is the problem? I can think of at least two things:
First, there's a tendency to want to be rigorous, sophisticated, and scientific, and this leads philosophers to invent technical concepts and apply them in situations where they serve no purpose. Take, for example, the concept of a possible world. A possible world is basically a possible situation, except that its emphasized that it is a "maximal" situation, including or excluding every detail that the world might possibly have. Sometimes, it's a useful concept. Sometimes you want to imagine a hypothetical world exactly like ours, except for a short list of very specific changes. Or sometimes, you want to imagine a world with a very small number of objects and nothing else. That's all okay. (Or at least not as bad as what I want to complain about here.)
However, philosophers have fallen in love with the concept of possible worlds, and begin invoking the concept in situations where it's useless or counter-productive. Situations where, most importantly, they aren't really concerned to have every detail fixed. For example, last semester in philosophy of mind we encountered a theory of the relationship between mind and matter (I think it was called "global supervenience") that was explained in terms of possible worlds. It turned out that an immediate consequence of the theory was that the position of a hydrogen atom in a distant galaxy might be vitally important in determining our mental states (thanks in part to possible worlds emphasizing the idea of every detail being fixed). And this is a theory which philosophers had seriously put forth. They never meant to say something so absurd, but did so because they got using technical concepts when they didn't need them.
Another example in this problem is adequately summed up in the second quotation in this post. Basically: philosophers wasting a lot of time on the nature of a certain claim, when all that mattered is whether it's true.
The other thing that's silly about philosophy: philosophers taking themselves way to seriously. For example, in the metaphysics class I'm taking right now, we basically spent over a week discussing the transporters from Star Trek. This was done with minimal self-awareness and irony. Now, debates on Star Trek message board about what this or that piece of technology would really be like can get quite heated (or so I've heard). However, Trekkies are at least capable of keeping the MST3K mantra stuck somewhere in the back of their minds, even if it's at the bottom of a chest in the attic of their brains, with the path to said chest blocked by a pile of lamps, coat hangers, and bicycles. The point is, it's there, they know it's just TV. In philosophy, however, there is no equivalent to the realization that it's just TV. It's serious business. And that makes all the discussions feel very off.