This is a personal view, but I think the big differance is in the leagues.
In Spain, the teams are technically gifted, and play an open attacking game. More importantly, the weaker teams play an open, passing game too, which makes the whole thing more entertaining, and lets the big teams play ball. When the little teams win, they do so because they have played better football on the day.
In England (for example), the weaker teams are taught to play deep, and to use the long ball combined with tough tackling to combat the big teams' strengths. This make the bigger teams in England more attack-minded too, and they learn how to pick open stubborn defences. This is why they can be more effective against Italian teams than the Spaniards.
The style of play makes Spain more attractive to South Americans, as do the language and the weather. Real are the traditional team everyone loves to hate, and Barca are their traditional rivals. Hence, South Americans love Barca.
The Dutch connection has already been mentioned, and has been in place since Cruyff signed for them in the 70s. Other Dutch players followed him, and a number of their managers have been Dutch. Van Gaal's team was a virtual Dutch enclave, as half the Dutch national team was lining out for Barca. So Dutch kids love Barca too.
Throw in the huge stadium, packed with 100,000 very noisy and passionate fans, their history of playing good football, and the fac that they represent a repressed nation to their fans... whoever said it first was right - it's more than a club.