config.wyam

#n Wyam.Markdown -p
#n Wyam.Razor -p
#n Wyam.Yaml -p
#n Wyam.Minification -p
#n AngleSharp

#an "System.ServiceModel, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"
#an "System.Xml, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"

Settings.Host = "hyr.mn";

//Process each post as /post-sluggified-name/index.html so we get what looks like extensionless pages on gh-pages

Pipelines.Add("Posts",
	ReadFiles("posts/*.md"),  // Read all markdown files in the "posts" directory
	FrontMatter(Yaml()),  // Load any frontmatter and parse it as YAML markup
	Markdown(),  // Render the markdown content
	Meta("Post", @doc.Content),  // Move the markdown content to metadata
	Merge(
        ReadFiles("posts/post.cshtml")  // Load the Razor post page template
    ),
	Razor(),  // Compile and render the page template
	Meta("PostFile", string.Format(@"{0}\index.html", ((string)@doc["Title"]).ToLower().Replace(' ', '-'))),  // Use the post title as the filename and save it in metadata so we can use it in links later
	Meta("PostLink", string.Format(@"{0}/", ((string)@doc["Title"]).ToLower().Replace(' ', '-'))),  // Friendly link for post
	WriteFiles((string)@doc["PostFile"])  // Write the post file 
);

//I want the most recent post to be the home index as well.

Pipelines.Add("MostRecentAsIndex",
	ReadFiles("posts/*.md"),  // Read all markdown files in the "posts" directory
	OrderBy(@doc.Get<DateTime>("Published")),
	Take(1),
	FrontMatter(Yaml()),  // Load any frontmatter and parse it as YAML markup
	Markdown(),  // Render the markdown content
	Meta("Post", @doc.Content),  // Move the markdown content to metadata
	Razor(),  // Compile and render the page template
	Meta("PostFile", string.Format(@"{0}\index.html", ((string)@doc["Title"]).ToLower().Replace(' ', '-'))),  // Use the post title as the filename and save it in metadata so we can use it in links later
	Meta("PostLink", string.Format(@"{0}/", ((string)@doc["Title"]).ToLower().Replace(' ', '-'))),  // Friendly link for post
	WriteFiles((doc, ctx) =>  "index.html")
);

//Process the rest of the top-level pages

Pipelines.Add("TopLevel",
	ReadFiles("{!_,!index}*.cshtml"),
	FrontMatter(Yaml()),
	Razor(),
	WriteFiles(".html")
);

//Combine and minify the css assets
Pipelines.Add("css",
	ReadFiles("css/*.css"),
	MinifyCss(),
	Combine(),
	WriteFiles((doc, ctx) =>  "css/style.css")
);

//Copy over the other layout assets (images, font)
Pipelines.Add("Assets",
	CopyFiles("css/*{!.css,}")
);

The other ones like OrderBy didn't click for me though.

You've got the call right. I would expect the first try of yours to work:

OrderBy(@doc.Get<DateTime>("Published")).Descending(),
Take(1),

I'm assuming this is in relation to the "MostRecentAsIndex" pipeline above:

Pipelines.Add("MostRecentAsIndex",
    ReadFiles("posts/*.md"),  // Read all markdown files in the "posts" directory
    OrderBy(@doc.Get<DateTime>("Published")),
    Take(1),
    FrontMatter(Yaml()),  // Load any frontmatter and parse it as YAML markup
    Markdown(),  // Render the markdown content
        // ...

The problem is that at the point where you're calling OrderBy the front matter hasn't been processed yet, so there isn't a "Published" metadata key in the documents to order by. In that case the delegate that gets the metadata value for OrderBy is returning a default value, which is the same for every document. Change the pipeline to this and it should work:

Pipelines.Add("MostRecentAsIndex",
    ReadFiles("posts/*.md"),  // Read all markdown files in the "posts" directory
    FrontMatter(Yaml()),  // Load any frontmatter and parse it as YAML markup
    OrderBy(@doc.Get<DateTime>("Published")),
    Take(1),
    Markdown(),  // Render the markdown content
        // ...