What makes a good bootleg? The best and most compelling mashups aren't just A+B, two songs smashed together or even two songs put together that match acoustically. It's when two songs come together to give you an entirely new perspective that a true mashup is born. Madonna and the News does this, 99 Luft Balloons nails it, Yeah Yeah Yeah I Want Your Love, and scores of other greats.
When asked to drill down to humor's core, comedians will tell you that there are just two simple ingredients: recognition, and perspective. Audiences first recognize something being referenced, then it's made funny by giving it some new perspective. That's it.
So I think the best tech mashups aren't too different. Yesterday I ran into @jonwheatly's Twitority. Simple in that all it does is search Twitter but adds new perspective to the results, filtering by the number of followers the users making each statement have.
In some important ways, that's all creativity is: looking at something existing in a new way. We shouldn't ask anything more complicated out of truly great ideas. What's the great/simple/creative idea you've run into recently?