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Pro Free and open source
Pro Easy setup
Vanilla is a MySQL + PHP app and can be set up very easily.
Pro Discussions can be organized in categories and tagged
Discussions can be tagged with an arbitrary number of tags, separately from categories. Categories are hierarchical and don't scale; tags do.
Pro Importer from many other forums
Vanilla Porter can import posts (comments), threads (discussions), users, categories, roles and passwords (hashed) from most other forum software: bbPress, Drupal, MyBB, NoteBB, phpBB, PunBB, SimpleMachines 1&2, Toast, UserVoice, vBulletin, Xenforo, YAF.NET and others. Some importers support avatars, attachments, signatures, and PMs. A few support tags and user notes.
Imports aren't flawless, but they do most of the work.
PHP may not be the most modern language, but Vanilla is fast.
Pro Hosted and self-hosted options
Pro Simple forum software
lightweight Internet forum package
Import from other forums
Pro Single Sign On
Ability to sign in with another system to your forum
Pro Deep integration with WordPress
Use SSO with your WordPress site, adds recent discussions widgets and best of all have comments also create discussions in your forum.
Con Poor search, unless using Sphinx
Con No post edit history
Old versions of a post are only visible to admins, which may lead to a credibility problem. The amount of time during which users can edit their posts is configurable from a few minutes to indefinitely.
Con No sitemap for discussion
Even sitemap plugin cannot add discussion in xml file.