Well, although Brian and Jorgen have described it so well, I'll bite too. Several things come to mind.
1. third party lenses
For many people the point of mirrorless (at least partly) is to be able to use legacy and third party lenses. The OMD really nailed this with the Image stabilisation working so well on a half press of the shutter - meaning that one can easily manually focus a 300mm lens from the Olympus glory days (or leica or whoever). You can't do this as well with any other mirrorless camera.
2. Sensor Quality
µ43 has been plagued with the mediocre Panasonic sensors - the Sony unit in the OMD has transcended the problem, producing fantastic image quality, even at high ISO. If it has an AA filter it's very light, and the images are sharp and sparkly
3. Auto Focus
It's incredibly fast - the continuous is not good, but the one shot AF is just incredible - I've used it for surfing shots with a 600mm equivalent lens and nailed pretty much every shot. The touch focus/shoot together with this incredible focusing speed is really good for both tripod mounted macro shooting, and unobtrusive street shooting.
4. Body, ergonomics and size
It's a camera - and it feels like one. Sure there's infinite depths of menus if you want them. But it works extremely well as a PASM camera, without ever going in to the menus - it handles really well, added to which it's small and easy to carry - if you want it bigger then put on the grip . . .yes - it's part of a system
As for the poll in dpreview - there sure were a lot of people who voted - nobody would suggest that the OMD is the best selling camera of the lot, so it's not just people supporting the camera they bought. If it were just going to be fanboys boosting their own purchasing decisions then the OMD is not the winner you'd expect (and the D800 certainly not the runner up). I tend to trust thousands of users more than a reviewer!
From a personal point of view, and having watched the poll develop (and thus seeing the also rans and their votes) I would have said that it was a pretty good summary of the year's releases (close runners included the Fuji cameras etc.) It's a pity they don't seem to publish the whole list.
all the best