I fully agree. KLM used to be one of the best up until the early 1990's. Ever since the close cooperation with Northwest started, they started to go rapidly downhill. I do about 25 flights on KLM / year (a mix of European and inter-continental flights and a mix of business class and economy class) and fly a lot on many other airlines, too, and KLM these days is about average. They have never lost my bags (LH and CX did lose my bags), I haven't had major delays, the food is normally OK (nothing special, but certainly edible).
The fact that KLM is doing relatively well in financial terms and has recently reported high load factors, also is proof that they can't be too bad.
I think that's very unfair. Yes, we used to roll our eyes about classy KLM getting involved with Northworst. But the last few years, something has happened. I can't remember anymore when I last dealt with a Northwest employee who was anything less than correct, and I can remember plenty of recent examples of good and friendly customer service, and plain Mid-Western decency. I certainly can't say the same about KLM. Northwest have managed to make quite substantial cuts whilst keeping their employees motivated and their customers at least content and usually satisfied. Compare this with KLM's infuriating puritan Calvinist ideas that their customers don't *deserve* ice with their soft drink, a beer, or (most recently) bringing more than 23 kg of luggage over the pond (why, anybody who has more than that must surely be too attached to their worldy possessions, which is why KLM usually tries to part them from their owner during transfer at AMS).
Sorry for the rant, which is off-topic as this thread is supposed to be a plain list of worst carrier without chapter and verse, but I don't think it's a coincidence that KLM is the most commonly mentioned one, and I also don't think it's reasonable to blame Northwest for this.