The high reliability of Linux is often attributed to the lack of traditional debugging tools commonly available to other software projects. The defect rate of Linux is remarkable considering there are few debugging tools available. However, the reason it is so low is more likely because the lack of debugging tools prevents people from getting involved and fixing problems, so only the experts can fix problems. Moreover, attributing the high quality of fixes to lack of debugging tools does a disservice to the level of expertise of those working on Linux, and to Linus' ability to recognize and apply a good fix when he sees it. It would be hard for anyone to break out a single reason why Linux bug fixes are the way they are.
New developers can join open mailing lists and participate immediately with seasoned hackers.
But because Linux is a volunteer effort, it must face the risks inherent in having developers with widely varied skill levels working on it. Thus, limiting changes to the kernel has a stabilizing effect, and keeps the risk of damage done by donated code at a minimum. However, even if non-experts aren't allowed to submit code to the Linux kernel, they should still be allowed to hack on their own kernels, and they should have the tools to do this expediently. One of Linux's success stories is the kernel source code review process. It's not perfect, but it does contribute significantly to the overall consistency of the kernel, moreso than I think the availability and style of debugging tools does.