To me, world and campaign building are the meat and potatoes of role playing. I am a homebrewer through and through. I hardly use any published material, as a matter of fact the amount of RPG books that I own can fit neatly on one small shelf. On the other hand the amount of homebrewed material that I have wouldn't fit neatly into a large room. Creating worlds and campaigns is what I do best, and its what I love most about gaming.
I have used many game systems over the years, but I find myself becoming more indifferent to the system with age. I've always found that you can argue the virtues of a game system until the cows come home, but the system isn't worth shit without great adventures, cool campaigns and an awesome world to host them in. You can always change the game system you're playing, but you can't turn off the suck from a terrible campaign. For me play trumps system any day.
I like to build my gaming worlds with as little detail as possible to begin with, usually just a map and jot notes about the overall concept. I like to be able to sum up the entire world in three paragraphs (I've also found three paragraphs to be the maximum mental retention for the average player... I'm being facetious, but only slightly). This allows for evolution during play and gives plenty of freedom for my players to rock out with their characters.
For campaigns I create the meta plot first and just let the players do what they want, working out the details as we go. I have found that allowing my players ambitions in the game to be more important than my own ambitions in the game, always results in an awesome campaign. This doesn't mean that I don't spin a good yarn when we play, it just means that I let them "play" their characters the way they want, and I facilitate their experience in the game world... and sometimes punish their stupidity, which is also quite fun.
My in game play style is what I would call organized improvisation. I prep minimally and leave lots of space for development as we play. I know the world and the campaign, but I don't know where the players will end up over the course of a game session. This might terrify some gamemasters, but I love it. I'm playing the game too, and improvising keeps me on my toes and playing right there with the PC's.
That's how I play the game, what about you?