The basic idea of Goethe's motto "without haste, without rest" is that you should do nothing thoughtlessly or carelessly - never be in too much of a hurry or act without thinking - but also you should never be idle and always strive to be doing something useful or good.
I agree with wordy in many ways but I think the meaning goes far beyond it. There is a much more emotional side to what he is trying to say. He does not say, "when working..." he is talking about a general attitude we should have towards life. It reminds me a little of Buddha's Middle path.