It would be smaller, lighter and cheaper. A smaller image circle requires less glass, less glass will get by with a weaker (lighter, cheaper) AF system. Fewer materials reduce weight and price. Compare a 80mm f/2.8 medium format lens (Hasselblad, Pentacon Six) to a 80mm f/1.8 35mm SLR lens, the size difference is noticeable.

And that's why it probably won't get made.

This is a common problem a lot of people who discuss cameras on the internet fall into: they want cheap, premium-quality accessories for lower-level gear. Canon has it's market pretty well divided: cheaper cameras for people who are willing to go with a 1.6x crop and more expensive, higher-end cameras for people willing to pay the non-crop premium (the 7D being an exception to this). I will say this plainly: if you didn't bother to buy a high-end body, you will probably never buy an expensive lens. Most crop sensor camera buyers never spend more on lenses than they do on bodies.

EF-S/DX lenses only really make sense when a) dealing with focal lengths that are unwieldy on full frame and b) creating cheap kit lenses that have to be small and cheap. Great examples of a) are the 17/18mm-50/55mm 'kit' lenses made by pretty much every crop sensor manufacturer. A 17mm lens on full frame is extremely wide and requires a lot of glass to get anything close to a high quality image. Compare the size of a 17-40 f/4L to a 18-55 kit lens. A good example of b) would be the 55-250mm. It's a tiny, cheap lens. Compare it to the 100-400 f/3.5-5.6L - a tremendous difference.

The thing with the 200mm f/2.8 is this - by creating an EF-S model, Canon would likely sell a few more lenses than just by having an EF model. The problem is, it would cannibalize sales of the more expensive, higher margin EF lens. Most people who want a 200mm f/2.8 prime will just buy the EF, the number of people who'll forgo getting it just because it's not an EF-S lens is incredibly small. To create an EF-S version Canon would have to lay out money for R&D, promotion, etc. They would create a lens that many people would avoid, thinking "if I ever upgrade to full frame, I'll never be able to use this lens."

Generally speaking, Canon telephotos are excellent, expensive pro lenses. You need to be serious about your photography if you're willing to drop 5 grand on a 300mm f/2.8L. I doubt there are many people - and I'm sure Canon will agree with me here - who look at a $5000 lens and say, "if this was only $4000, I'd buy it in a heartbeat".