The more classes I design (and many people would say the answer to that is "too many") the more I find it an interesting challenge that always presents new surprises.
One thing that fascinates me is how the design process for a 20 level class, a 10 level class and a 5 level class are so fundamentally different.
You'd think that you could design a 20 level class and lop off the first 5 levels, and voila! 5 level Prestige Class.
In reality though, each type presents its own unique challenges.
Oddly (to me), the harder classes to design are at the ends. 20 level classes because you want them to be "lifetime" classes. When I design a core class, I want the player to wince when he multiclasses. I want every level to seem valuable enough that he wants to stay.
5 level classes need to be like a laser. Get in and get out. What is this class? Why do you play it? Its like a mechanics poem.
More on this later Im sure...