I just brewed my second batch of beer, and of course totally freaked out at the slow start. My first batch had started bubbling the airlock after 2 hours, and had huge krausen and constant bubbling a couple of hours later. By the end of day 2, it was all done. The outgas smelled like fruit cocktail.
This time around, with liquid yeast and slightly lower temperature (70ºF), I had nothing -- no airlock bubbles, no krausen, no anything -- for nearly two full days. The first night, the airlock level actually went backward. But finally, this afternoon, I got some positive pressure and bubbling (still no krausen, but I'm learning to relax and not worry). It smells much cleaner than last time.
While I was waiting, I freaked out about yeast temperature (I let it stay at room temperature too long), aeration (did I shake the bucket long enough), pH issues (I heard something about pH and assumed that I had accidentally reached some kind of deadly acidity).
So, for me, all the advice about relaxing for 72 hours turned out to be true. I wonder how many times it'll take before I can actually stop worrying...