Email participants of personal messages.
Show 404 page in production when ActiveRecord::RecordNotFound is encountered.
Transmit the HTTP error codes to the client
Add atom feed
Mark threads with new replies as unread
Massively speed up mark-all-as-read
Use post_url helper for construction post urls
Add posts pagination
Make quickreply form an ordinary non-js form.

This works just fine without JS and is more reliable.
Don't query each watcher separately, use single SQL query