When you look closely the only thing that is the same on the feature list is the approximate number of megapixels. The mkIII is an entirely new camera, new type of chassis, new viewfinder, new shutter assembly, new button layouts, new software. Nothing has been recycled, unlike the mkII.
the higher FPS shooting, and the dual storage to SD Card are nice I guess
The obvious omission here is the new AF system, which is significantly better in spec to the 5D mkII, which had effectively the same 8 year old 9 point AF of the 5D. Many people were extremely disappointed in this when the 5D mkII was released, feeling that AF was the only real weak-point of the original 5D.
In fact the AF system is nearly identical to that of the flagship 1DX, utilising a 61 point AF sensor with a total of 41 'cross' type sensors (sensitive to detail in two directions). In comparison the 5D mkII has only 9 AF points, one of which is a cross type. The only difference between the 1DX and 5D mkIII AF is the 1DX has a much better metering sensor with its own dedicated DIGIC4 processor, allowing it to send colour information to the AF sensor to aid subject tracking. the 1DX can also make use of face detection to assist AF.
Any situation which places heavy demands on AF such as shooting fast objects, shooting in low light, shooting off-centre compositions with fast lenses will see a significant improvement. For some this will mostly justify the $1300!
Another criticism of the mkI,mkII was the slow shooting speed which has been upgraded to a respectable 6fps. Not a deal breaker but nice.
For anyone shooting weddings dual card slots can be a lifesaver, and easily worth $1300.
Video has seen some improvements, to moire and noise, and the codec. However resolution hasn't improved, for many the 5d mkII would be a better choice for video until magic lantern arrives on the mkIII.
Whilst improvements have been made in this area, mainly with respect to noise the quoted one stop improvement in high ISO shooting is only attainable in JPEG mode, and is partially achieved by the use of stronger noise reduction. Consensus among owners is that shooting raw the improvement is less than half a stop. The 1 megapixel increase isn't really noticeable, and was done to allow 3x oversampling for moire reduction in video. So image quality wise, it's a only slight improvement, nothing like the jump in resolution the mkII brought.
Whether it's worth it to you is hard to answer, if you don't find the AF on the 5D limiting then you may want to save your money and get a mkII.
Effectively the new model produces images of similar quality to the mkII, but is likely to produce more keepers and be more pleasant to shoot with.