Shifting from the specs for a moment. One thing that is always overlooked when discussing the hype and rumors surrounding upgrades is the simple fact that Apple is running a business and they do not want to put all their eggs in one basket.
Like any manufactuer, Apple wants repeat business. They not only want new buyers, they rely heavily on existing customers to upgrade their device, preferably at each iteration of the product. You cannot do this if you fill an upgrade to the brim with all of the features that are in demand. IMO, even if it were possible, it would not be a smart business move to put a retina display, 4G wireless, an A6 processor, and the most popular software updates all into the Ipad 3. From the perspective of the business, why would you? As it stands now, based on history, it will sell like hotcakes regardless of what you put in it. It will sell even more with a prime upgrade such as a high-res display. That's all you need. If you throw down all the cards, you will not give current users any substantial reason to want to upgrade a year from now. Such a device would ensure that the purchaser is set for the next 2-3 years before it becomes an obsolete device with outdated specs and features. You want to entice customers with a couple of new features but you do not want to give away the farm.
Apple has a limited range of products. The majority of their revenue now comes from the Iphone and Ipad. You do not want to jeoapordize future growth with these limited products by being too slick with the current iteration. You want to keep people hoping and dreaming about the next tieration. Another reason why all of these features will not all appear in the Ipad 3, unless Apple hired some really bad planners lately.