The killer app for a Bitcoin exchange would be...
The killer app for a Bitcoin exchange would be to always show the correct "Last price" on their home page, a feature that I would like to see come to MtGox soon:
Will the Gox / CoinLab integration also include this "feature?"
It's funny you say that, the data feed issue is a big one, actually. All I can say is we're working hard on this with Gox, and I feel your need.
A programmer can fix this in 30 mins or less, it's not something one needs to work 'hard' to achieve. Only use USD for calculations, not other currencies.