A major difference is that web content does not have silent access to your microphone, and the ability to monitor you and send information back home.
There are large security differences between what NaCl allows for web content, and what it allows for what Google considers internal parts of Chromium, even if they run in NaCl. Like this blob here, and also Flash, the PDF plugin, and others. All those do a lot more than what a random website would be allowed to.
There are large security differences between what NaCl allows for web content, and what it allows for what Google considers internal parts of Chromium, even if they run in NaCl. Like this blob here, and also Flash, the PDF plugin, and others. All those do a lot more than what a random website would be allowed to.