I would distinguish between two types of question here:
- Questions answered with a variation of
\usepackage{spam}
or another well-documented solution (e.g. CTAN, TUG-UK, DANTE or what have you). In any case, a brief explanation should suffice, provided that a link to the e.g. package documentation or the authoritative solution is given. There is little incentive (or benefit, for that matter) for anyone to repeat what has already been done elsewhere, and probably explained better and in more detail.
- Homegrown solutions: basically everything else. Here I'm talking about stuff like arcane TeX wizardry, specific bibliography request, creating special TikZ diagrams and so on. The answerer should be expected to provide more details on parts of the solution if/when requested, but that shouldn't extend to "go through the code and explain every line in a 5-sentence paragraph". I'd imagine that in most cases a brief explanation of the tricky part should suffice.
A somewhat tangential suggestion: new TeX users often find it challenging to contribute back to TeX-SE if they are not very well-versed in the typesetting system yet. I've seen at least a few users which were interested in participating, but were frustrated that they cannot even understand what some of the questions are about. Encouraging them to expand on a question that can be answered in a few lines, going from \usepackage{spam}
and a CTAN link to producing a full (not necessarily minimum) working example will go a long way to retain them as regular users in the future. They will also build up some reputation this way, and as we all very well know, this is the perfect gateway drug into TeX-SE.
In this way TeX-SE could get both quick answers (usually by some of the old-timers), which are later elaborated into a more comprehensive example by users new to TeX for a long-term benefit. This could benefit old questions with already accepted answers as well.