Like the Mac's beep, these simple indicators save users a lot of grief. Certainly systems with CRTs or other alphanumeric displays can easily show lots of useful error information. Working in C makes formatting output especially easy. Use these resources, but don't depend on them. Learn from the automotive companies.
Cars have a lot of sensors, all wired to an under-hood computer.
Ford, GM, and others let the mechanic put the computer into a self-test mode, and flag errors by toggling one bit very slowly. The engineers cleverly realized that a voltmeter is about all you can count on a mechanic having and understanding, so their software drives the bit up and down so slowly that even a meter needle can show the transitions. A LED is just as effective and even easier to use. If the product is too cost sensitive to include even a 50 cent LED, provide a place to clip one on. If you use a LED rather than a voltmeter, than the flashes can be quite a bit faster.