Question: The question mould is:

> 
> How does X work? 
>  I've read the [*Wikipedia article / original paper / documentation / other resource*] but I can't make sense of it. Please explain X to me in plain English. 
> 
> 
> 

where X is a concept, algorithm, pattern, etc. (Not a code dump.)

I've come across a number questions fitting this mould. I never really know how to react. The result could perhaps be quite useful, but it doesn't quite seem to be a natural fit for Stack Overflow as a Q&A site. 

One reason is, it isn't clear what the appropriate scope of a good answer would be:

* Explain which part? (Some of it? All of it?)
* To what level of detail?

The appropriate result is ill-defined and unbounded: could be anything between a one-liner and a 100-page treatise. 

Example\*:

* [How does Dijkstra's self-stabilizing algorithm work?](https://stackoverflow.com/questions/6435046/how-does-dijkstras-self-stabilizing-algorithm-work)

Questions:
----------

* What is the appropriate reaction to such a question?
* When is such a question valid?

\* Feel free to edit this if you want to add more examples.

Answer: 
The part of the question that makes the biggest difference for me is whether the user is asking for help understanding a *topic* or a *specific resource*. I love questions that say "I have read through resources X, Y, and Z, but still don't understand this concept". Those usually get an upvote from me, provided they're formulated well.

If the question is inverted and instead says, "I have read this resource, and it covers concepts X, Y, and Z. Can you explain the whole document to me?" I will usually flag those as too broad.

You've specified in your question that X is a "concept, algorithm, pattern, etc", so that would fall into the first category I described, and I think those kinds of questions are great as long as they are specific and list at least a few resources that the OP has attempted to understand.
The questioner should indicate which portion of the topic he already understands and which not. Otherwise it's not clear from which point the answer should start. I would refuse to answer "please explain X to me" type questions unless the questioner also mentions specific points he/she doesn't understand.

If this is clear and you don't think the answer would be too long (too broad) or can be found elsewhere (not enough research) then why not writing a great answer.

In the specific linked case the specific question is "I still don't quite get the k-state solution." So one could maybe give an answer detailing the k-state solution (if it is not too long) and then adapt the question towards "How does one obtain the k-state solution in Dijkstra's self-stabilizing algorithm?" if one wanted to.