How complete is BitDefender's protection for Windows 8?
The level of protection is adequate. However, you should keep in mind that windows 8 comes with quite a few protection features by itself. The system is much more robust and harder to crack, compared to its predecesors.
You will have Windows Defender (upgraded to provide full protection), Windows Smart Screen (which checks executable files against a database of known malware) and Windows Firewall. Bit Defender has its own version of each of the above, and has done so for quite some time (except for Smart Screen). There might be conflicts, although if you turn off all Windows built-in features, there shouldn't be any issues.