I have time for only a quick note and a link this evening, so I will make this short and to the point: If you are experimenting with custom kernels, or if you are new to building your own kernel and would like some sort of guide to the configuration process, you might try out the guide here, on wikia.com.
I haven’t been through every page of course, so I can’t be sure it’s 100 percent up-to-date, but a lot of the explanations and rationales are useful. It’s quite clear-cut about which parts are generally safe to trim, and which parts are probably essential. Most of it appears to come from the kernel help pages themselves, but there are parts that are clearly annotated by an outside source.
In any case, it’s worth looking at if you are new to The Big Show, or if you are an old hand and just want to review your own notes. If you know of any other kernel guides (my own not included ;) ), feel free to clue me in; I make a habit of jotting down references for the future.