• .njk as the default templating language is an odd choice, and I find myself stubbing my toe on it a good amount. Maybe that's a me thing!
• It is extremely fast. This site has around 2300 pages; Eleventy is compiling it in around two seconds.
• The extensibility ergonomics are terrific. This is all it takes to declare a new filter, for instance:

eleventyConfig.addFilter("toStars", (rating) => {
  return "★".repeat((rating + 1) / 2);
});

• I can keep the Markdown content relatively pure, which (after poor and brief dalliances with .mdx et al) is now Very Important to me.