There still could be something wrong with your feed. Run it through this validator:
http://validator.w3.org/feed/
If it's valid, then the problem is definitely with Google Reader.
If it finds any errors, and they aren't specific to content within a particular post, try deactivating all plugins. If that resolves the issue, reactivate each one individually until you find the cause.
If that does not resolve the issue, try switching to the Default theme (WordPress 1.5 - 2.9.2) or the Twenty Ten theme (WordPress 3.0 and higher) to rule-out a theme-specific issue (themes can affect feeds).