There has been quite a lot of talk lately regarding bodies and where they are going, especially with MP, Diffraction, and image quality. Given the length of time within this profession, I thought I would give my 2 cents regarding these issues...
Ever since Canon and Nikon started making digital DSLR's with the D30, D60, and 10D for canon and Nikon's first digital DSLR (that I'm aware of), the D1x, the D30 was 3 MP and the D60, 10D, and D1x were all 6 MP. At that time photographers criticized the cameras and MP and resolution because it maxed out at just under an 8"x10" print resolution. Keep in mind photo printers needed and required at that time 300dpi to get anything photo quality, so photographers lived and died by that resolution. It was determined by people much more technical that me that the resolution of the top slide film was in the low to mid 20mp's and so Canon took notice and slowly started edging up the MP to reach that holy grail.
Now that the 1Ds and 5D mII has reached that holy grail, canon is thinking on how they can push the limits beyond film and produce something within the medium format realm which I would welcome as long as the image quality doesn't suffer from where it is already. Nikon hasn't been so aggressive in the MP race and has suffered in the early stages of digital photography because of that. Nikon has recently started playing the same game with canon regarding MP however they have a slightly different approach with only umping the top tier cameras of each class leaving plenty of room for upgrades and division between classes of photographers. Canon on the other hand tries to save money by having fewer sensor option trickling down sensors (1Ds to 5D mII, 7D to 60D to Rebel T2I, etc... ) I'm sure they would trickle down the 1D sensor if they made another camera that format size.
There are photographers who say "I dont need X amount of MP! Nikon only makes 12 Mp and that's all I need" Well congratulations, you find yourself in the majority of all photographers in the world. While I have my 5D and 7D, it is not often I print larger than 11x14 and not too often I print 11x14's at that unless there is a reason such as someone is buying it or I'm shooting a commercial shoot for a client. Back in the days of film, with full frame 35mm's, most people printed 4x6's, 5x7's, and the occasional 8x10's. Photographers rarely ever printed 11x14's with film negatives because the quality would be slightly compromised. Lab technicians at film labs would question the photographer to make sure they wanted that size. Not much has changed from now to then in that regards except even fewer prints are printed and instead put of digital picture frames, facebook, flickr, and email. You hear the argument that the difference between a 70-200 2.8 lens and the 70-200 F4 lens is that the F4 can never go to F2.8 if and when you need it. Same goes to MP... you may rarely need to use the sensor at full resolution and down-sample in post production everything you shoot, however if you have a low MP camera, it will never quite be the same upsampled as if it was shot with a higher resolution camera. It is really as simple as that.
Lastly, diffraction. Yes, that is a pain in the butt. With that said, lens, especially entry level lenses, have not changed dramatically from the film days in regards to quality. The common rule of thumb with film was you should spend twice the amount on lenses than the body. This wasn't hard because bodies were so simple and relatively cheap. With digital, lenses, for the most part minus inflation, have remained pretty stable in pricing with bodies double, tripling, and in some cameras, skyrocketing compared to their film counterparts, so people now try to skimp on lenses because of the high prices of cameras. Earlier I said in the film days, people rarely printed 11x14's let alone anything bigger cause of image quality issues. If you take a 11x14 printed from a film camera 35mm, hold a lupe up to it, you would see the exact same diffraction limits as a 7D at full resolution at 100% with an entry level lens. The difference between now and then because people used to not dare print at that size and now since things are so cheap and its quite easy to throw images in photoshop and zoom to 100%, it's much much easier to notice now. This falls in the same realm of you get what you pay for. This isn't anything new. This isn't something that magically appeared because light hits a sensor rather than a slightly curved plane of film. Its just much easier to dissect an image now than it was 10 years ago. Plus I would also dare to say there is double to triple the quantity of "professional photographers" than in the film days because it's easier than in the film days. The technique and prices have overall lowered with the lack of dark room, and it is easier to learn. The limitations are there and will not be going away so we as photographers need to raise the bar, learn to compensate, overcome, and enjoy photography. People who complain about current cameras are the same who complained about the D30, D60, and 10D. Some people will never be happy, however they are also important because if it wasn't for them, there would be no need for Canon and Nikon to improve.