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?"
Already explained this a bunch of times, but I will again. The last price shown on MtGox is the price for the last trade, no matter which currency. If someone buys a lot in another currency than USD, he'll end paying more (in his own currency) than the highest price in USD, but will not cause the highest USD price to move up.
I explained this quite a lot already, and the ticker API provides various varieties of last price.