a) disposable dust bags. I do not want to mess about emptying dust chambers. I want to be able to easily lift out a full dust bag and drop it in a bin. There, - job done.
b) for it to be as light as possible and with a comfortable lifting handle for ease of carrying it around the house.
c) for it to have a power lead that automatically rewinds for storage.
d) for it to have easily dismountable extension tubes and tools.
Let's get this straight. I could not care less if the suction drops off as the bag fills up. I don't measure the amount of dust in the carpet before and after cleaning and even if I could and did and was worried about it, I could just keep cleaning for a little longer or clean more often.
Does anyone agree with me here? And does it perhaps make you wonder if the success of a certain very popular make of cleaner that looks like a miniature chemical processing factory, and has made its inventor a small fortune, might not have been solely due to the public's gullibility?