Okay, Spain:
Regular involvement in Govt. decisions: regional elections every four years, general elections the following one. That's all. It's quite limited, and most people don't exactly know what they can really do.
I'm well-informed (I read two newspapers and watch regular news programmes on TV), and I do believe people need to be well-informed (hell, I'm not studying journalism for any other reasons), but sadly people just listen to their regular "preachers" and just repeat some propaganda as a mantra. And then they go watch football, Big Brother or some other crap, because "politics are so complicated/boring/whatever", and I believe it's really sad. Oh well.
And I'm not a fan of "secret agendas", I just know people in great corporations want to make as much money as possible and they just try to convince the politicians to help them. The main difference between parties is the level of involvement of the government in stating regulations and giving social rights to the citizens. But in a capitalistic world, the guys with money have the power to invest on whoever they want to support the politicies they want, so there isn't much to choose from out there.
