Rule 3: Don’t Delay Gratification
This rule is implicit in Rule 1, but it deserves explicit mention. A class shouldn’t be designed such that members of that class are underpowered at low-levels and have to ‘pay their dues’ in order to access the actually-cool abilities later. Delayed gratification isn’t any fun during the delay, and in many games it isn’t clear that there will be a “later” in which you will get to reap the benefits of your dues-paying.
On gaming forums, classes are often compared with the assumption that the character will be level 20. This makes little sense to me: in the vast majority of campaigns, even if the character reaches level 20, most of its play-time will be far below that. Moreover, not all characters are played to level 20 – or even to level 3. Keep this in mind when designing the class.