Hi Rory, Well, I just went the other way from a Olympus E620 which is a 4/3 to the Nikon D7000. My main concern was what to do with the excellent lenses that I own for the 620. They work on the E-5 (OM-D EM-5!!) with an adapter, but there are issues when doing that. Anyway, I was at a discount day at Hunt's camera and the Nikon and Olympus dealers were side by side. I had a D5100 in my hand when I asked the Nikon dealer why he would prefer the Nikon to the Oly and his answer was two words, "The sensor". He also pointed out that with the Nikon there would be a much wider choice of lenses. Nikon accessories are more widely available, too. When I lost the remote shutter part, stores had the Nikon, but not the Olympus in stock.
What drew me to the Nikon D7000 was the focus options. The E-5 has a interesting touch screen, however. Just touch the screen on the spot where you want the focus point to be an it appears!
What I will miss on the Nikon is the swivel options on the LCD, especially when I have the camera held down for a low angled shot.
In the Oly, I also got used to seeing the exposure number in the LCD in Manual Mode. It was easy to change the EV number and watch the effect it would have on the image. This can be done with the D7000, but the exposure button has to be pressed and there does not seem to be a connection between the viewfinder exposure number, the screen on the top, and that on the LCD screen. The Nikon is filled with many more features than the OLY which will take longer to learn, I am sure. The Oly manual fits easily in a pocket, which was convenient for the field while I was learning it.
I will miss the lightweight and size of the Oly. The mm on the Oly is double that of the 35mm which works to one's advantage with telephoto, but not wide angle. For example, the 4/3 300mm equals 600mm on a 35mm scale, but a 12mm 4/3 equals a 24mm.
The only reason I changed was that the OLY has been having to go for repairs more frequently including a vacation last month when I had rented a high quality wide angle lens from BorrowLens that I never got to use. Groan. (I have had the camera over three years, but it was not new when I purchased it.)
Anyway, those are some random details that do not have much to do with the quality of images. I certainly do not have the ability to distinguish which cameras were used in any photo I have seen in contests here, or in courses I have taken online!
Downhill skiers have a wide choice of brands and models to choose from, but I rarely, if ever, hear someone complain about a new pair of skis. People tend to enjoy what they own, just like photographers.