I've been a laptop-only user for almost a decade now, but I'm thinking of building myself a desktop soon so I can have more power. While doing some preliminary research, I came across the old-ish SU question Is it still cheaper to build your own PC?.
In the spirit of making questions less localized and more about general learning — per Jeff and others at my recent Meta question — here's a new question: What factors should be considered by someone building a custom PC? Just as importantly, what factors aren't that important?
As a subquestion, how do these factors change based on the intended use of the computer (e.g. software development vs. gaming vs. photo editing/graphic design, &c.)?