GNU Hurd
I've had an interest in the Hurd operating system for a while - it's a microkernel based OS being created from scratch, which is an interesting undertaking in itself. I've always been interested in OS-level advancements, and this seems like the most promising place for really innovative stuff.
But this note scares me:
The Hurd can only support partition sizes of up to approximately two gigabytes; anything larger than this will not work. This limitation is due to a design decision that was made several years ago in which the filesystem server maps the entire filesystem into virtual memory. As the amount of virtual memory available on an ia32 is only four gigabytes of which Mach allocates three gigabytes to the application and, of that, a significant portion is reserved for the code, the stack and the heap, the final, maximum contiguous virtual memory area that remains is generally about two gigabytes. This limitation is scheduled to be removed.
This seems astoundingly short-sighted, and makes me wonder what other short-sighted decisions have been made.