I schlepped a Nikon system around much of the world for two decades, and while I appreciated its qualities, I now mostly just shoot for myself. When I made that transition, one of my first priorities was finding a lighter system. I tried a Leica M8, and though I had most of the fast lenses, I found that it didn't really suit me in the end. The long end was a 135mm (effectively a 180 with the M8's crop factor) and it was essentially a low ISO camera -- you start to run into deteriorating images at about ISO800. I never went above 400. Some of that is made up by the fast glass, but the fast glass is terribly expensive. On the other end, macro work is difficult because of parallax problems. I think you could pretty much forget bird shooting with an M8...Going back to the cost of the glass, I got a check for about $25,000 from the consignment company when I sold mine off. For $6,000, you could expect a used 8.2 body and one used lens.
After the adventure with the Leica, I stayed with Nikon for a while, shooting a D3 and a D300, which are a nice pair, but heavy. I then tried the m4/3, eventually buying three cameras. I can get all three cameras -- a GF1, a GH1 and a GH2, two chargers (the GH2 and the other G cameras used different sized batteries), four lenses, and the ancillary equipment in a briefcase. One thing I *really* like about the GH cameras is the twistable LCD. You can almost use them like a Rolleiflex. Or, if you're trying to be somewhat covert, you can actually shoot backwards, if you know what I mean...point the lens in one direction, while you are looking at the LCD in a different direction. You can get along quite well with three lenses, that will take you out to an effective 600mm. They are really good cameras. But: the IQ is not what you'd get from a FF or a Leica 8.2. The sensor size is about 1/4 of FF, about half of an APS-C. From reading this forum, I believe that Michael Reichmann is spending the winter in Mexico, and took along nothing but a Panasonic GH system.
Still looking, I mostly recently tried a Pentax K5, and I think I will be there for a while. It's notably smaller than the Nikon system, and the lenses are smaller, too. It is fully weather sealed, has really good high ISO performance, has an effective 16mp sensor, and a decent lens selection. For the size of photos I print, I don't lose much to FF. Although it is notably lighter than the Nikon, it is also notably heavier than the GH2.
Overall, I guess I'd suggest that you look at the Panasonics, but also at some APS-C cameras. Most of them are pretty big, but a few are very good but quite compact. If you don't need really big prints (bigger than 19") and do a lot of hard traveling, I'd pick up a couple of GH2s, or a GH2 and a GF1 with the optional clip-on finder, three or four lenses, and call it a day. You could easily put all of that in a small briefcase. By the way, Voightlander now offers an m4/3 mount 25mm (effectively 50mm) manual f0.95 lens that supposedly works well with the Panasonic. That's as fast as camera lenses get. Image quality is supposed to be good, and the price is unbeatable at $900.
JC