The weatherproofing really is impressive - I took an om-d 5 on a monthlong hike in vermont(most of the long trail), and it rode on my shoulder essentially the whole way (only ending up in its holster in the heaviest rain), 3000 images on that trip, and the little guy still works just fine! I now have that camera plus an om-d1 and a bunch of lenses including the stunning 12-40 (shortening Olympus' unwieldy names a bit), because I was so impressed with the om-d5 on the hike. I had the om-d1 out in a serious snowstorm for three hours yesterday, and the poor camera looked like a snowball (the hood was keeping snow off the front element, but I had to keep blowing off the viewfinder), but it kept right on shooting! It works fine today, too... Olympus claims that the weatherproofing is up to Nikon D4 or Canon 1dx standards, and i have no reason to doubt them, and quite a few to believe them...
That's roughly the range on the image quality, too - really good 16mp camera ( 1dsII, D4, D700, although it doesn't have the high ISO of the D4). No, it's not a 36 mp camera, but it offers darned good quality up to 20 x 27", and is wholly acceptable at 24x30". Sure, if you have a printer the size of a piano, you can get extra resolution out of a D800e ( there is no question in a huge print, although I will be darned if I can tell in a 16x20 of most subjects, including high - detail landscape).
There are two basic schools of landscape and nature photography - the perfectionist who will accept serious limits on where they can shoot ( the older Ansel Adams), or is exceptionally strong and agile, plus has help to carry gear ( the younger Ansel Adams, whose wife ended up with the tent and food many times - fortunately, both of them were really strong), and the photographer who says the best shot may not be somewhere the biggest camera is going (Galen Rowell).
If you're doing fast and light backcountry landscape, the Om-d series is one of only two really viable travel companions (the new, weatherproof Fuji X-t1 looks awfully intriguing as well). You won't always have a tripod, so the superb stabilization on the Olympus bodies (and several of the Fuji lenses) will save your bacon on a frequent basis. The cameras can take anything the Weather God can dish out, and both systems have some superb optics (although we haven't yet seen Fuji's weather sealed lenses, I'm confident they'll be great, having shot extensively with a variety of X-series lenses). I can carry an Olympus body, the 12-40, the superb 60mm macro lens and the Panasonic 100-300 ( a surprisingly decent, although not great, super telephoto) in a small waist pack! What other system offers a 600mm equivalent wildlife lens in a package the size of a soda can? Even if it's a merely decent lens, it goes places a 600mm Nikkor can't dream of, and doesn't cost as much as a decent used car.
The obvious question is "what about the Sony A7 and A7r - they're not much bigger than the Olympus"? First of all , the Sony bodies have FOUR native lenses (two zooms with pretty much the same range, and two primes with focal lengths within the range of the zooms, none of them a macro lens). The Zeiss zoom looks like an awfully nice lens, but it makes every other lens for the system redundant (in a real sense, they have one native lens...). I would almost guarantee that there is enough slop in adapters that my om-d bodies with the native 12-40 (or an x-t1 with a good Fujinon) will out-resolve any adapted lens on a Sony, with the possible exception of a Leica lens on a very expensive adapter - critical focus is a matter of fractions of a mm, and the tolerances of most adapters aren't great. The Olympus or Fuji shooter will have the choice of excellent autofocus or manual focus with automatic focusing aids, plus image stabilization, while the Sony shooter has to focus the adapted lens manually, turning focus aids on and off by hand, and has no stabilizer. The weather sealing on the Sonys is dubious - look at Roger Cicala's recent tear down - he couldn't find any gaskets, although there are quite a few flanges, which are better than nothing. Sony might yet do wonderful things with the A7 range, but I don't think they're there yet.
An exciting time to be a backcountry landscape shooter, with two options Galen Rowell would have killed for...