Ok, take UAC [at least on Vista/Weven]
This is designed to limit apps to standard user privileges, unless an admin authorises the provileges to be elevated.
In this way under normal operation, the OS will prompt you if an app [that included a browser], tries to do anything is shouldn't
[run executable code, access the registry, access other running applications etc.]
So on a domestic client pc, the OS provides this, but also provides the user with the ability to override, it's there choice.
[If the OS just simply stopped things, imagine the complaints, the OS doesn't really know the difference between malicious and not, so it simply hands the choice to the user, assuming said user has sufficient privileges to run as admin etc.]
Now we all know that not all web sites are nice, and some will try to run malicious code etc.
So, when you run your browser as administrator, it gets a slightly freer reign on what it can do, meaning a page could try to run something without you knowing it. Yes, anti-malware, virus checkers, and the browsers themselves have things in place to detect and prevent.
All that I'm saying is that having to run your browser as admin removes a layer, leaving you more vulnerable than you would otherwise have been.
Not everyone would run the browser as admin just in order to run Mythos, thay may use that same browser session to trawl the internet.
I've had to deal with the fallout of people that havn't taken appropriate precautions, it's not pretty.