1) content streams (RSS)
2) muscle and connective tissue (APIs)
3) pretty skin (AJAX)
But there's one element that seems to be missing from the mashup body out there: intelligence. Jason Douglas made this clear for me when he said that at some point these mashups are going to have to get smart about who they're serving. Just because you can mix something up doesn't mean it should be or that anyone will care.
Zillow, the ultimate real estate mashup, is an interesting case study. There are several dials in the user interface that make it very easy for me to research what I want. The depth is incredible.
But Zillow should be smart enough to identify what kind of researcher I am by my behavior. Am I browsing for fun? Am I drilling into a neighborhood looking at specifc sized/priced properties? Am I watching the value of my own home compared to the market?
There are lots of different ways to optimize for different types of users. If you get that right then the money will follow.
For example, advertisers want to offer different things to different users at each stage of the purchase process. The purchase process for anything typically looks like this:
Awareness -> Research -> Comparison -> Transaction -> Loyalty
Zillow should have ways to optimize for and capitalize on these different audience segements. If I'm browsing for fun, then perhaps I also want to know what the local council has to say about living in the area. If I'm drilling in, perhaps I also need to know who the good agents are or even how to buy without one. If I'm watching my own home, maybe I need someone to recommend an architect or a good local contractor for that remodel I've been planning.
The audience segments for the service could also be drawn using a subscription model. For example, if I'm a real estate agent with a particular skill at helping buyers, then I'm going to fear for my job now that Zillow takes away my advantage. I'd gladly pay Zillow a hefty fee to keep it out of the hands of all the people who are using it for free today or to at least give me special reporting tools that I can use to advise the buyers who aren't clued in yet.
It's the character, intelligence, and the behavioral wrapper that the tool needs to put around the mashup that will help it stand out in the crowd. Without it, the typical mashup legacy may only be inspiration for a smarter company who will make the idea actually work for people.