I've been a fair few times and I have to disagree in some ways as there is plenty of passion from large areas of the ground. I agree in the respect though that there are always a lot of tourists or day trippers who attend and they expect to turn up and see a Harlem globe trotters performance and become quite critical when or if they don't get it.
Hum, i´ve never been to Camp Nou so i have no proof but i believed that the non-tourists are the most demanding and the tourists just wanna take photos and stuff.
In Fc Porto the amount of whistlers is disgusting. I´ve seen whistlers start booing in the 1st minute just because of a bad pass... And Porto has been an overachiever in the last 30 years. lol