I want to build a http://usertesting.com/ like site. I am thinking to build it on WordPress. Will it be ok to build a startup on top of WordPress?
It really depends what you are doing. Wordpress is the most widely used web platform for brochure and ecommerce sites. It also can be used for heavy content sites of course. http://usertesting.com/ is a platform build on JS and likely not a good candidate for wordpress - you would want to build something custom to imitate this site.
Its an easy start as long as you are not using any online transactions .
Go with WP if you want to test out your product market fit or concept because you can get it up and running quickly depending on your user and business requirements.
To build a site similiar to usertesting.com using WP you stil need to make a lot of changes on the backend to make it work. If you have the money / or time, try to build out more robust solution on your own.
My answer is yes. There are a lot of successful business people with Wordpress sites. You have to be a careful with security and know the ways to protect yourself.
Here are some examples of extremely successful businesses and people with wordpress sites; both informational and e-commerce: