Actually...this is based on some slightly outdated information, but basically, the creators have said that each bending style has a sort of special style that is accessible to only some members of the style.
The special styles as given were:
Earth: Magnetism (that thing that the Dai Li were doing with their gloves, and Bumi spider-climbing walls)
Now, based on the way it was described, and things I've gathered from the show, I have a couple theories:
1) Your ability to use these techniques is only tangentially related to how "good" you are at bending: that is, you could be the strongest firebender in the world, and still not know lightning bending, because lightning bending is *different*, and requires a fundamentally different state of mind from firebending.
2)Things like Metalbending, Blue Fire, and Bloodbending aren't special styles, so much as they're just something that incredibly strong benders can do.
Metalbending might be the exception there. It might require one to be incredibly strong but also know that magnetism thing, or it may be that the creators have changed their mind about what Earth's special style should be. I dunno.
As for airbending, the special style could be bending a new form of the element (vacuum does sound interesting, but I agree that it would be awkward for a kid's show) or else something that is traditionally related to the element, like healing is to water. Something like precognition. I'd have to look up what attributes tend to be related to air.