Subversion vs Mercurial

It has linear history, central repository and management. Lacks too complex features, almost every developer is familiar with it, so everyone knows what to expect and how to work with it.

Gives you a better indication on how old a specific revision is without needing any more details.

E.g. more easy to understand than Git's submodules.

Doesn't require access to a central repository, because each checkout is a full repository by itself.

Mercurial is a more intuitive option than many of its competitors. The documentation is well organized and easy for beginners to read & understand.

Mercurial is open source and free to use.

Mercurial is designed to make performing operations (such as commits, clones etc.) fast.

Mercurial documentation covers everything from revision control system basics to mercurial specific features in a short and clear way.

Users can activate plugins to provide additional features.

The command-line interface is more intuitive, requiring much less memorization and exotic options/piping to achieve the same thing you would in git.

Native Windows support. No trace of cygwin or other dirty hacks typically found in competitors.

Mercurial makes it pretty darn hard to delete history by mistake. In Git, if you mistakenly commit to HEAD and switch to a different branch, your commit is toast. Yes, you can dig through the reflog but most users don't even know of its existence and will wish they didn't once they find out.

Mercurial is actively contributed to by the developers at Facebook as they use it for their massive source repository.