For reduced space we can communicate that there is information about the topic in a compact way, and provide access to the whole description for those interested.

I created a quick prototype and some mockups illustrating the idea:

I like this. I think it could be even more compact than this -- the top of most of the pages is a box that says "This article is of interest to the following WikiProjects", which isn't worth excerpting.

I think the way that we'd calculate the number of "notes" is to count the number of templates, unless somebody has a better idea.

With the option to edit the header I was expecting people to write some general introduction, but from the examples I have been checking, I see they are template-only. For those cases, I agree that a more compact option would be preferred (since a short text taken from the templates won't be a great summary of the contents):

If we really need to make the entry point more prominent depending on the content (e.g., if some templates are marked as really important for users to read) we can use the background color in a similar way as notifications (highlight it until the user opens it for the first time):

This is great; let's do it. I'll put the mockup at the top and then on to the estimation meeting. :)