It is obviously in the interest of review sites, and I cannot claim innocence in this, to exaggerate differences between cameras and the models that replace them, but as time goes by I'm becoming more convinced that things stay the same more than they change. Sure there are oddball cameras that do show a significant difference, such as the Sigma DP2 Merrill and the Leica M9, but these can't really be described as "mainstream" cameras for everyone. Neither is particularly easy to use and both are poor at higher ISO's and will remain very much as "niche" cameras.
I believe Chris is also right when he stresses the importance of getting the taking of the image right and the importance of post processing in determining just how good the image will look. The first thing I do with a new camera is work on my raw conversion to see what are the best settings to achieve optimum quality. It is also vital to get exposure right, to make sure the camera is stable and to use a decent lens, though I have always regarded that as the least important.
Newer is not always better, at least in terms of IQ, but many would have us believe that there is a steep upward curve in what digital cameras are capable of producing. But it probably isn't. However this does have one great advantage, in that you can pick up "last years model" at ridiculous prices. If you can stand the shame of not possessing the latest and greatest, then most times you are unlikely to compromise your photographic output or your bank balance.
Picture at the top of the page is taken with a Panasonic GX1.