I trade on MtGox. There is 0 chance I will trade on an exchange that charges 3% each way. Just to enter and exit a trade, it's already a 6% hit.

I used to be a market maker on Bitfloor when it was around. Bitfloor actually paid liquidity providers for each transaction, rather than charging. That's the way to add liquidity to your exchange, especially when you're just starting.