This thread nicely sums up why I switched from UA to NW. On UA, my family and I experienced several long delays (including two overnighters), cancellations, and mis-connects, and we never received anything. Not one bonus mile, not one cent in travel vouchers. (Not until we complained to the DOT about UA's violation of its contract of carriage, that is. In that case we got travel vouchers three months after our flight.)

One reason I switched to NW was that I had heard from friends about their policy of offering compensation (bonus miles, etc.) in such cases. So far, in six years of flying NW, I've never had a big delay that would call for such compensation. But it's nice to know that NW often does this for its passengers.