As it has been said many times in this forum, it depends on the user's browsing habits.
If he is the type that clicks like crazy any link he sees in his browser then much more protection is required (or even nothing can save him from being infected).
However, if you are a careful browser with common sense (and some security add-ons installed in your browser), the built-in security of Win8 + some on-demand virus scanning will be enough.
In my opinion it completely depends on the user and will always vary user to user, no form of security is bullet proof and even layered configs (or tho being stronger) are not impenetrable.
I'm a Windows 8 user since release date and my security consists of the default Windows Defender, Windows Firewall, Windows SmartScreen and Windows UAC and if you apply all them with some brain cells to not click on unknown links or run unknown applications/setups without doing background checks such as URLVoid, VirusTotal and Anubis you'll be close to fine as I've had not one single infection as far.
Doesn't mean you let your guard down, I always have complete system backups made and other rescue methods in hand incase a little nasty some how manages to get by but that's very rare and hasn't happened to me in a long time.
So does Windows 8 need an antivirus? Yes.
Are you absolutely required to use 3rd party tools in order to obtain adequate protection on Windows 8? No.
The first video test is completely flawed. When Smart Screen and UAC warns you to not run the file the reviewer runs the files anyway. What kind of a test is that? Your your AV blocks a malware would you disable it to run the malware anyway? if so then you deserve the infection.
The second video is more accurate but still don't show the complete picture. If you don't always download files from trusted sources then you will get infected no matter what type of security products you use.
I currently use Windows Defender + Windows Firewall + UAC on my Windows 8 laptop and I have never had one single infection.
Of coarse I always watch what I do, so does Windows 8 default security offer enough protection for novice users? well no it doesn't and none of the other third-party security products will do any better at keeping novice users safe.
Your actions are far more important then what security setup you use.
Is Windows 8 default security enough protection for users who watch their own actions when downloading files and surfing the web? Yes it is because UAC will block any type of threat if you use it correctly and only approve known trusted programs.
Exactly, Littlebits. The PC Security guy said you can tell those were malware just by looking at the links - just a bunch of scrambled numbers/letters, followed by .exe. Whilst driving a car, it's called Defensive Driving.