These questions are very broad and seem as if you don't really know what you want to do.
My aim is to build an exchange that is more focused on what the customer wants, as opposed to making a profit. I'm aware that that is the general aim anyway, but I think more focus on CS is necessary in this environment.
Not to be negative but running a trade/exchange is a huge amount of work and takes a lot of experience. I would shy away from this if you don't know much about the technical/law/PR/etc
I can do the GUI and backend no problem. I have a working knowlege of law in my country, and as far as PR goes, I'm just going to be brutally honest with people. I have no qualms about telling people if I have a problem, I'd rather my customers be able to make a correctly informed choice.
we really need a good exchange that trades between the crypto currencies
You're right. That is a good idea, but in practice it would have to allow for small deviations in the correct stock prices, since the blockchains all seem to update at different times. It's workable though.
If a public service exists, someone is going to complain about it.
Yup. The very least a service proprietor can do is listen to and respond to those complaints. A lot of these exchanges don't seem to do so.