I am running into the problem with requestValidationMode="4.0" where if you submit html tags in a form the request will be marked as invalid and the app will throw A potentially dangerous Request.Form value was detected from the client.
The two most popular solutions are to use requestValidationMode="2.0" combined with validateRequest='false' on either a global level or keeping global as 4.0 but making a subdirectory that is listed as 2.0 in its web.config and putting all the pages that you don't want validated there. 
What I would really like is to keep 4.0 but add a little logic to the 4.0 RequestValidator class to prevent it from throwing an error if it's just HTML in a form.
 
    