After hearing speculation about Fluttershy being part Earth Pony I have to wonder what these traits applied to other ponies would be. Here's my speculation.
Pegasus with Earth Pony: Have a stronger connection with plants and animals than Pegasus and are slightly stronger with will that can accomplish some feats(Fluttershy's "stare"). Weaker flier on average.
Pegasus with Unicorn: Can use certain amount of magic tied to the force of their body limited by cutie mark. (for example having a grip that ties to elemental forces like say holding water if the pegasus has a rain related cutie mark). Activates more magic at high speeds but less stable in their flight abilities.
Unicorn with Earth Pony: In average they're far more durable than pure unicorns but they rely on full contact magic. Generally they need to touch the object they intend to enchant.
Unicorn with Pegasus: Capable of walking on clouds and manipulating air currents and liquids with telekinesis (correct me if anypony was ever shown using telekinesis on a non-solid) but have a much lower maximum weight when lifting anything telekinetically.
Earth Pony with Unicorn: Able to take the "earth ponies can do anything if the work hard enough" mantra
Up to Eleven. Are capable of doing the impossible with enough effort (breeding animals that possess traits that cannot exist naturally, making trees grow out of horsemade objects) but generally slower than their companions.
Earth Pony with Pegasus: Can walk on clouds and are faster on average, with an ability to leap far higher than pure Earth ponies but are far weaker and less able of using Earth ponies talent with plants and animals.
edited 7th Oct '11 1:23:42 PM by RhymeBeat