Treantmonklvl20
Explorer
While I enjoy Treatmonk's guide, the most useful guide I've seen (that was for another game) was a step by step walkthrough of a character's life. In it, the author walked through character creation, advancement, etc... and talked about the decision making process whenever the PC made meaningful decisions. Within it, the context of decision making was evident - which is something that is hard to account for whe you're just talking about whether something is good or bad.
This is a good idea.
Now I think it's hard to come up for general rules that apply to all builds, but taking a single build from level 1-20 and discuss the decision making at each level up I think is very good. Of course, those decisions change based on the class and specifics of the build, but that guide could at least show you the thought process at each level.
90% of the guides I start writing never go anywhere, usually because I can't come up with a good way to organize them in a meaningful way. That said, I've been working on a concept where I take 3 builds, and follow a process similar to this. I was working on an archery guide that would follow a "Stealthy woodlands archer" an "arcane casting archer" and a "hand crossbow dpr specialist" over 20 levels.
However, maybe I might start with a guide that makes 3 more varied builds. A God Wizard, and maybe a Hexblade gish and something else. I'll have to give it some thought...