The best feature of WordPress 2.0 is definitely the addition of Akismet. This comment spam blocking plugin has single-handedly made blogging fun again. Not having to deal with hundreds or thousands of junk spam comments gives you more time to write and post worthwhile content. If you are upgrading, Akismet comes with WordPress 2.0, but you can also check out Worst Offenders, which makes it easier to identify your worse spammers, by giving you a list of the top offenders and their IP addresses.