I think people forget that airlines the size of NW (or AA, or DL, or UA, for that matter) are bound to drop the ball sometimes. By the same token, they'll deliver stellar -- if not satisfactory -- performance more often than not. Also, I reeeeallllly think a lot of the traveling public tends to bash airlines based on what they've heard others say. If they've heard and read from six other people about "how horrible" airline X was, then that person has got to come up with his own anecdote about that "horrible airline," even if he has only flown it once and the service was sufficient.
NW has just been the trendiest carrier to bash over the past few years. You'd be considered odd if you were to say NW was your "favorite airline."
Though I work for UA, I take NW quite frequently (non-rev of course) only because I have relatives in both Detroit and Minneapolis. (NW offers frequent nonstops between those cities and the NYC area, which UA doesn't do.) I must say that contrary to what I read & hear, about 7 out of 10 flights are actually AS GOOD as any other airline. The cabin crews are excellent, and the planes' interiors are ALWAYS clean. (Customer service and gate agents could use a little more people skills though.)
BTW, this forum seems to specialize in frequent UA bashing and an unusual degree of DL praising quite often.
Now this I really don't understand!