I would have to agree with many others who say that price is critical. Obviously, that is partnered with the quality and number of inputs/outputs. A splitter typically only has one input and multiple outputs, so I would think that keeping a fair price as the number of outputs increases makes sense.
Most people though, won't need more than 2 outputs.
What would I love to see? I think it would be great if the splitter would act as a D/A and actually allow for a full 50 foot HDMI run after the connection. So, if a HDMI signal is sent along 30 feet to the splitter, it could then be sent another 50 feet beyond that point. So, signal amplification seems like a golden addition to this type of product.
Size? Meh, small works in the back of my rack, rull 19" with rack ears in the box works at the front. I don't care which. But, I would hope that it would comform to rack spacing size standards and be available with some sort of rack mount option.
Also - my opinion - HDMI! DVI is great for the PC crowd, but give me HDMI for my video. I want the built in audio and higher quality potential thanks.