The 'no fun' principle

I've heard it whispered and seen it printed of late, something that greatly befuddles me. It was leveled at The Dark Knight and I've seen it utilised in a few early reviews of the new Bond film Quantam of Solace. These films are great, but are not quite as fun as they should be. I'd love to know what they mean by that, should they be easier to take in? Less hard-hitting? Should they be more forgettable, or is a big film simply not allowed to take itself seriously and thus their lack of bludgeoning irony is their crime. Fun for fun's sake is great, but isn't it a good thing that blockbusters are making an effort to be better films rather then just exist to bring in the money. And its not exactly if these films aren't entertaining, just not cheaply so. The beef then can only be with the darker, more adult tone. Which not to tell the people what to think is a very very very good thing