Wowpedia is a new wiki, officially separate from WoWWiki but considered a successor site in all but name. It is not hosted by or in any way affiliated with Wikia, the site which hosts WoWWiki and refuses to transfer the domain name.
Origins
Due to irreconcilable differences with Wikia, the WoWWiki community resolved in October 2010 that it would leave Wikia. However, it turned out that the domain name wowwiki.com had been legally transferred to Wikia upon WoWWiki joining the then-nascent network in 2007. As Wikia refused to relinquish control to the domain's rightful owners, a mirror site was set up, and Curse offered to host it.
WoWWiki contains a complete copy of articles from WoWWiki as of 18 October 2010. If you choose to leave WoWWiki, any work you did on WoWWiki after that point may need to be redone on Wowpedia, although it's likely it has been updated by someone else.
Differences
There are a number of differences, some imperceptible to the lay user. The most obvious are that the sidebar is in its original place and there are no advertisements. Many of the new 'social' features introduced by Wikia do not exist, such as achievements and avatars. Additionally, your watchlist will remain private. (Most users are probably not even aware that watched pages are now 'followed' according to Wikia terminology, and are visible to the public.)
However, the content of the site is the same, as is the community that runs it.
Reception
The decision among WoWWiki's administrators was unanimous. Virtually the entire active user base (as well as some new faces!) has transferred to Wowpedia as of the time of this writing.
Some critics have noted that the new Wowpedia site has had trouble staying online at times. This is because the setup was rushed as they wanted to have it up in time for Blizzcon
Reclaiming your user name
Wowpedia contains a tool to 'reclaim' one's user name. All you have to do is click on the 'Register' button and tell it your user name. The site will then generate a code; you can enter this code in an edit summary on your WoWWiki user page to confirm for the site that you are who you say you are. You will not lose any of the edits you made prior to 18 Octoboer 2010, as the entire history up to that point (including your user page and any subpages) is intact.
Note that, while you technically have the option of staying on WoWWiki, the site will likely become outdated rapidly due to the lack of a user base.