Top 8 Web 2.0 Security Threats - ReadWriteWeb
http://www.readwriteweb.com/archives/top_8_web_20_security_threats.php

Programmatic interfaces of Web 2.0 applications let hackers automate attacks easier. In addition to brute force and CSRF attacks, other examples include the automated retrieval of a large amount of information and the automated opening of accounts. Anti-automation mechanisms like Captchas can help slow down or thwart these types of attacks.

Google's worst-case scenarios.