I bought my Nikon D300 almost exactly 3 years ago. It's been a great camera and I'm still shooting great images with it. From a commercial standpoint, people are still paying me to shoot with it. So here's a big "thumbs up" to you D300. That being said, it is 3 years old (ancient by tech standards) and I have been looking to upgrade for some time now. Upgrade?? Wait, hold on....you just said it's a great camera shooting great images getting great use? Why would you want to upgrade? Nikon has several sensor sizes in dSLR bodies: DX (APS-C, a "cropped" sensor) and FX (full frame). The main difference between the two is size. According to Wikipedia, Nikon DX sensors come in around 23.6 x 15.8 MM (give or take) whereas full-frame FX sensors come in around 36 x 24 MM (about the size of a 35mm film negative, sensor size comparison here). The main advantage of the larger FX sensor is better low light performance and dynamic range under certain conditions. The "crop" factor of the DX sensor can come in handy sometimes too, especially for telephoto. Having around a 1.5x crop factor means that 24mm lenses act like 36mm on DX, 200mm acts like 300mm, etc.
So when I first bought into dSLRs, I got the D300, a DX camera. I shot a lot of pictures. I also started getting paid for jobs. After awhile, I wanted to upgrade to FX as well as get a back-up camera (I think its stupid to go into jobs without a backup cam). Last year I had money to do this. My plan was to buy an FX camera and use the DX D300 for backup. Big problem. The FX cameras in Nikons lineup. The D3x and D3s were too expensive for my needs at the time. The only other FX option, the D700, was over two years old at that point. I didn't want to drop $2500+ on a two year old piece of tech gear. So what did I do? Bought another DX camera that was cheaper that the D300 (the D7000) that actually has better specs on paper than the D300. The D7000 has been great (esp the 16MP resolution vs 12MP of the D300) but it just wasn't exactly what I was looking for. The D700 replacement was really what I wanted.
There have been a lot of rumors about a D700 replacement lately. Supposedly it will be called the D800. Nikon Rumors posted purported pics here. The specs of the camera have generated a lot of buzz on the web, and here are my two cents:
1) If those pictures are to believed, the body size and control layout represents a departure from current prosumer and pro bodies. It more resembles the D7000. I'm not too crazy about this. While this will make the D800 smaller and lighter, I like having all the buttons at my finger tips and am not a huge fan of the D7000 layout. Smaller and lighter is great, I guess, but the lenses I've got weigh a ton in my bag. Shaving a couple ounces off the size/weight of the camera won't make a huge difference to my back or Really Right Stuff tripod/ball head I will be ordering in the coming weeks.
2) 36MP would be awesome! A lot of people argue you don't need that many MPs. I disagree. The lenses that I have bought for my DX cameras (24-70 2.8G, 70-200 2.8G, 24 f1.4G, 105mm 2.8G) easily out-resolve the DX sensors. I'm more than happy to see FX sensors with more MPs that are going to be able to take advantage of the resolving power of the lenses I've already spent thousands of dollars on. Also, with more MPs comes a greater ability to crop. It's not always possible to get the composition you want and sometimes you have to crop. Cropping a larger file will give higher resolution crops. That is good. Bigger files also mean I better start buying higher capacity memory cards and hard drives.
3) Price...a little high but certainly doable esp with those specs. For the haters: granted the sensor is larger, the Leica S2 is currently the only weather-sealed camera in a dSLR form factor that can deliver that high MP images (37.5MP). The problem? It is $23k. The lenses are all in the $5k-9k range each. So 300,000 yen ($3900) for a D800 when I've already got some kick-ass Nikon F-mount lenses seems like a bargain. If you think it's too expensive, you don't need one: buy a $1200 D7000. I have one, it rocks for DX. 16MP, useable images at ISO6400, and 1080P 24FPS video for a fraction of the cost. Or buy a used D700 for FX. People will be selling lots of them when the D800 comes out.
4) A lot of people are complaining about AF and FPS. I don't think Nikon is going for the sports or bird-watching market with this camera. This is not the camera for you if you care about shooting fast moving things at high FPS.
5) Video: good video specs, hopefully an 1/8" mini-jack for mic in (for connecting external mics/audio devices). If true, it will do 1080P at 24/25/30FPS and 720P at 24/25/30/60FPS. Just remember, if you're new to dSLR video you are going to need stabilization gear and continuous lighting. Don't expect to re-create Reverie just because you bought a dSLR that does video.
6) Memory cards: if it's got a CF and SD slot that'd be great. I shoot JPEG+RAW. JPEGs for quick client deliverables (or for delivery with no editing if they're good enough) and RAW for post. CF+SD would allow me to do RAW to CF, JPEG to SD at the same time. I'd be happy.
7) ISO: gonna have to wait to see samples to see how ISO stacks up to current FX and DX shooters.
Anyway, that's all I've got. If those specs and pics are too believed then sports and wildlife shooters may be disappointed but I'm pretty pumped. You can't make a camera for everyone but this looks like the next camera for me. Time to start raising money and all I've got to say to Nikon is: bring it on!