What have been the most important changes to LaTeX in the last five years? (2011–2016)

For me the biggest change is Lua(TeX): the ability to separate TeX's building blocks (processing the input / building the node lists / output). That way you can for example build a product that uses TeX's linebreaking and hyphenation algorithm and its superb output, but without its troublesome input language.

With the minimal TeX file

\catcode`\{=1
\catcode`\}=2

\directlua{require("whatever")}

\end

You can jump into Lua mode and access TeX from the Lua-side without switching back to TeX. The Lua end is powerful enough to load fonts and images, input files, process the input, and have comlete control over the output. There is a very nice and large API to do everything can do on the \macro{} level.

Be warned, this isn't an experts answer. It is an answer from an users perspective.

The whole ecosystem of LaTeX has improved a lot:

1. texlive comes with a package manager that really works (but I don't know exactly since which year). It is able to get specific packages from different repositories.
2. Inverse search and forward search work under Linux and Windows. Under Windows, SumatraPDF was improved a lot and around 2010 started to support SyncTeX as well. Under Linux and Emacs as an Editor you even can jump from a single letter in the PDF-Tools buffer to the letter in your source!
3. UTF-8 support in LaTeX as well as in the editors just worked.
4. Maybe someone can deliver more details, but I noticed a broad improvement with PDF. Errors with the colours in two column mode were corrected, no more errors with included, large PDFs and so on.
5. You can even get ebooks in epub format now, quite new packages.

LaTeX itself improved, have a look at the news at http://latex-project.org/ltnews/ . If we can conclude from the number of updates to the development, the LaTeX3 project is gaining momentum, see here as well: https://latex-project.org/l3news/ .

tikz and pstricks have seen many new versions. Some years back the package auto-pst-pdf made compiling pstricks pictures easier.

OK, and there were a gazillion of valuable improvements by new or updated packages. Let's take the margin column. S. Hicks came up with the marginfix package, which was superseded by the scrlayer-notecolumn package, written by M. Kohm. Now you can have marginnotes over pagebreaks.

There are many new fonts with pdftex support. Last week I discovered the gillius fonts, last autumn the imfellEnglish font. Never forget Linux Libertine...

I'm a user, unfortunately I'm still lacking programming skills. But from my point of view the improvement sum up to »works well«. A real new thing was the Emacs mode "org-mode", which got a new exporter to e.g. LaTeX.

A huge Thank you! to all the people who work to keep us happy with LaTeX \& Friends!

Tools like Pandoc.

Pandoc is able to convert Markdown to LaTeX.

I like LaTeX's output, but I'm not really fond of writing LaTeX files.

I guess more than 95% of the time Pandoc's Markdown is enough for me (esp. because it supports LaTeX math). In the end, the plain text input is much more readable than my former plain text LaTeX files.