I get the feeling you've never really used HAML, because this is a load of FUD. I'm going to respond primarily with HAML, because that's the whitespace-significant template language I have the most experience with.

> Writing plain old HTML is fucking simple. Abstracting it out into another 'language' just means you have to deal with a whole load of quirks and workarounds you'd never see with HTML, and while it looks all purdy and shit to some, it's an absolute nightmare to maintain.

I've maintained a lot of older projects, and the closer the templates are to HTML, the worse they seem to be. I can't count the number of times a DIV is left unclosed, or a TR is closed by a function call, or a P is nested 20 levels deep in a one-line monster tag. HAML and other whitespace-significant template languages prevent all that. You can't leave a tag unclosed in HAML, it closes them for you; because it closes them for you, you can't close them in weird and separate places; finally, because it forces you to have a newline before each tag, it looks really obvious when you've created a huge nested mess of HTML.

> Want to nest elements? Either you can't,

Wrong. Nesting elements is dead simple in HAML:

  %ul
    %li

yields

  <ul>
    <li></li>
  </ul>

> some syntax has been introduced to discourage it (the end of line pipe),

The end of line pipe is for when you have very code that goes with a single element. From the docs:

  %p= @this.is(way.too.much). |
      code("and I should").   |
      really_move.it.into(    |
        :a => @helper)        |

Usually, though, this should go into a helper, because your templates shouldn't have that much logic in them.

> or you're told to just... use plain HTML.

This makes me think you don't know what you're talking about. Again, it's easy to nest elements in HAML. You wouldn't drop to HTML just because you want to nest elements.

> Want to interpolate variables? That's probably built in, which is great until you try changing it back to plain HTML, and then have to scan for all the interpolation, which is now plain text.

Of course it's "probably built in," interpolation is core to any templating engine. Interpolation is dead simple, as well.

  %ul= some_variable

I'm not sure what you mean by "turn it back to plain HTML." At what point can you turn any template language back to plain HTML without having the interpolation be a problem?

> There's likely so much focus placed on "one-lining" everything that you have to do silly things just to actually make the template readable, when it wouldn't be a problem in HTML.

Do you have any examples of this? In my experience, whitespace-significant template languages do more to discourage one-line operations. Instead of doing this:

  <ul><li></li></ul>

you're forced to break it up into multiple lines:

  %ul
    %li

> Never mind the total lack of portability and, especially these days, requiring a copy of Node just to run the damn things. Which is fine until you realise you shouldn't need to add a full on javascript VM to your development dependencies.

I've never used a template language that didn't require some programming language to evaluate. Usually they require the language that the entire project is being built with; e.g., HAML assumes you are writing a Ruby project.

There are a lot of framework tools that require some sort of precompilation, too. If you're building anything reasonably complex, you're probably going to want to be able to split your files out into smaller chunks, which means using something like Less/SASS for CSS and require.js for dependency management. All of those have external dependencies. Or do you really not care to compile your disparate JS and CSS files into a single download?

> That so many of these attempts at 'HTML templating languages' exist is probably testament to the fact that no one agrees on what one should look like.

Isn't that a great thing, though? Diversity in this means the technology hasn't stagnated.

> They've actually been staring it in the face all along: plain old HTML.

I have no idea what you are suggesting with this point. What does HTML actually offer as a templating language? There's no built-in variable interpolation, no built-in template segmenting (e.g., rendering a partial), and no built-in control structures (loops, conditionals, functions, etc.). Once you've added all that in, you either have another template engine that requires something like node.js to precompile for performance reasons, or you have a mess of spaghetti that only the creator knows how to maintain.