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It doesn't necessarily have to be completely unique, but something that most MMOs simply don't have.
If you would your top three, please.
1. Jumping puzzles
I think this is my favorite aspect of the game that I haven't really seen anywhere before to any appreciable degree outside of Rift (and to a FAR lesser extent there).
2. Fast travel
I love this feature. I used to not mind the excruciating travel times found in other games, but I guess that I have grown old and somewhat impatient at traversing the same terrain over and over again.
3. Instant auction from inventory
Hard for me to think of a game with this feature, although it most assuredly (I hope) has been done before.