As an Argentine, what I see is those protests resemble a lot the ones we had on 2001, after several years of economic problems. Spain has an unemployment rate similar to the one we had back then (21%). The slogan of our protest was "Que se vayan todos!" ("Everyone has to leave"), referring to politicians. Spaniards are angry against their politicians, and they blame the two major parties (PSOE and PP) for most of their problems. The target of our protest also were the political parties. 10 years later, most of our political parties are still damaged from the fallout that ensued (we had 5 presidents over the next 2 weeks).
People compare these protests to the one on the Middle East, but I think it's very inaccurate.