It might help to predict what will work and what won't if you understand that I have two main concerns:

Security: Blocking things like script or Java applets or ActiveX controls etc. that pose a threat to users, because they run on their computers.

Vandalism: Preventing major modifications to the layout and design of the site in a way that might reflect badly on it.

IFRAMEs actually fall into both categories, because an IFRAME not only drastically modifies the appearance of the page, it introduces a hole through which script can gain a toehold. IFRAMEs are also used surreptitiously for things like hidden form submissions.