Although there are some attempts to move to a more modern microkernel altogether, the current Hurd implementation is based on GNU Mach, which is only a slightly modified variant of the original CMU Mach.
Unfortunately, Mach was created about two decades ago, and is in turn based on even older BSD code. Parts of the BSD kernel -- file systems, UNIX mechanisms like processes and signals, etc. -- were ripped out (to be implemented in userspace servers instead); while other mechanisms were added to allow implementing stuff in user space. (Pager interface, IPC, etc.)
Also, Mach being a research project, many things were tried, adding lots of optional features not really needed.
The result of all this is that the current code base is in a pretty bad shape. It's rather hard to make modifications -- to make better use of modern hardware for example, or even to fix bugs. The goal of this project is to improve the situation.
There are various things you can do here: Fixing compiler warnings; removing dead or unneeded code paths; restructuring code for readability and maintainability etc. -- a glance at the source code should quickly give you some ideas.
This task requires good knowledge of C, and experience with working on a large existing code base. Previous kernel hacking experience is an advantage, but not really necessary.
Possible mentors: Samuel Thibault (youpi)
Exercise: You should have no trouble finding something to improve when looking at the gnumach code, or even just at compiler warnings, for instance "implicit declaration of function", "format ‘%lu’ expects argument of type..." are easy to start with.