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.
"**Explain X to me"** is no different from "**Why does this code work/does not work?**". Your reaction should depend on whether additional information is included in the question, i.e. whether any research was made prior to posting this question.

I would argue that including links in the question constitutes research. You can google for your key phrase and include top 5 links, using 1min of your time.

Let's consider the question you linked as an example:

> 
> I have read his seminal paper, Self-stabilizing systems in spite of distributed control. However, I don't quite get how the self-stabilizing algorithm works. I am most interested in his, **'solution' of k-state machines**. The density of the paper is quite intense and I can't make much sense of it. How does this algorithm work in plain English?
> 
> 
> 

I highlighted the part which suggests that OP has in fact done some research. However, I think it is not enough to make a good question. Whether such a question should be answered is up to the community.

I would expect the OP to explain a bit more of what they understand, and what they don't, from the topic they want explained. My reasoning is simple - if they cannot provide a detailed explanation of what they don't understand, there is a high chance they will not be able to provide feedback on the answer (i.e. keep asking questions in comments, without ever getting to the point).