Most everyone agrees that the iPad 3 will increase the resolution of the iPad 2, from 1024-by-768 pixels to 2048-by-1536. This means sharper visuals on the screen, a special boon for those watching television shows or movies (might I suggest A Bug's Life?) on their tablets.
Past that, however, very little is known or has even been suggested as guesswork about upcoming iPad 3 features. Some have written that the iPad 3 will get Siri, Apple's popular voice command assistant, but this seems far from certain, and voice controls on a tablet seem far less essential and helpful than voice controls on a cell phone. There isn't even consensus yet about whether the iPad 3 will be thinner or thicker than the iPad 2 -- many reports have suggested that the hardware required to enable the improved display will necessitate a slightly thicker (0.7 mm, per iLounge) body.
One analyst -- Reitzes of Citi -- has even suggested that there might be TWO iPads released in spring 2012: An iPad 2S, with the same display as the iPad 2, and an iPad 3, with the upgraded screen quality. Both, Reitzes says, would feature a faster processor and Siri integration. The last time analysts claimed that Apple would release two new gadgets in the same line, of course, ended with with a lot of consumers waiting for an announcement of an iPhone 5 that never came.
Again, there are a lot of known unknowns and unknown unknowns surrounding the next iPad. Tales of total product overhaul and tablet revolution have been more muted than they were with the apocryphal iPhone 5, for what it's worth (very little); this update seems to be an iPad upgrade on specs rather than a re-imagination of the iPad's design and core functions.