I love the idea of writing a whodunit, but the whole point of a whodunit - they mystery of it all - can be frustrating as hell at times.
I find it easy to create characters. Coming up with a victim is the easiest. Creating a villain is also pretty easy (and a lot of fun). Deciding on a motive is usually pretty smooth (they basically all boil down to one of the seven deadly sins). And, choosing the murder weapon isn't too difficult.
What I find really hard is to figure out HOW the crime was committed, not just the moments of when it occurs (that can be easy or hard, depending on the story), but writing it so that the crime and clues to it are peppered into the story. The author should play fair with the reader and give enough clues so that a reader has a chance of solving the crime, but not in such a way as to make the plot a dead give-away. And the commission of the crime has to be believable, but cannot be too obvious.
So, once I get my head around that, then the writing goes very quickly. But until then, I stare at the computer screen, drink tons of coffee, pull my hair out and often lie awake until the wee hours of the morning.