[y4mi]

Just a heads up, a sister comment already pointed out the biggest "danger", but not what that means for your webapp:

Google will penalize your domain strongly as soon as anyone used your service for malicious content. You might even get blocked entirely if you are particularly unlucky.

That's also the reason why GitHub pages is hosted under github.io instead of GitHub.com for example.

[dharmab]

Safe Browsing is a must-consider for anyone hosting user-submitted content.