1. Does not have realtime protection, just a scanner.
2. Open source (I think the only one that is reputable)
3. Custom sigs
4. Configuration includes filters, so I'm not scanning text files or my .mp3 library.
5. Sigs are updated frequently
6. Free and sexy

I believe because it is open source, the more popular it gets, the stronger the scanner will be. Developers will be able to help out with signatures and be very quick with their releases. It is just a virus scanner and nothing more.