It uses a 47.6 bit key?

Generated by Math.random()? (According to MDN, that's seeded from the current time...)

Update: Another problem is that there is no message authentication. The server can flip individual bits within the message.

Update: Also, obviously, we have to trust noplaintext.com to send us an uncompromised web page.

But wait! View source!

<script type="text/javascript" async="" src="https://ssl.google-analytics.com/ga.js></script>;

We also have to trust Google, specifically google-analytics.com not to steal the message.

<script type="text/javascript" src="https://crypto-js.googlecode.com/files/2.5.3-crypto-sha1-hma...;

AHHHHHHHHHHHH!

We also have to trust whoever controls the crypto-js project and the people running googlecode.com.