1. Normal quotes (i.e. BTC/LTC instead of LTC/BTC, BTC/NMC instead of NMC/BTC, etc.)
2. Transparency
3. FIX Protocol
4. Liquidity Rebates
5. Professional trading platform
6. Margin trading optional (i.e. leverage, gearing)
7. Shorting (no naked shorting)
8. Every possible pair combination (i.e, BTC/LTC, BTC/NMC, NMC/LTC or LTC/NMC)
9. >99% server uptime
10. Free altcoins for registering.  

Edit: for clarity