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:
- 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.
- 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!
- UTF-8 support in LaTeX as well as in the editors just worked.
- 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.
- 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 S. Hicks came up with the marginfix package, which was superseded by the scrlayer-notecolumn package, written by M. Kohm.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!