HTML5 can power a new breed of web 2.0 websites. In fact, HTML5 will help move many websites and services into becoming full applications--more like those that run on your desktop.
With this new functionality, there are a slew of new web security concerns. From buffer overflow vulnerability with the canvas image-rendering feature to SQL injection attacks, the new features of HTML5 give alot more options to hackers and to those with malicious intent.
Browser makers and security teams are combing through the new functionality of HTML5 and it's security implications (NY TIMES: HTML5 Raises New Security Issues, August 20, 2010).