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.
Well, that part is done already at coinlab.com, enjoy. We don't try and convert other currencies to USD for quotes, though.