OpiWiki is a voting platform, discussion platform, ranking platform, and items database. We also provide lists and soon we'll have another key tool(s). Then we have a great category and tag system, unlike any other website. The title of this topic has been edited (from How is opi.wiki different than reddit.com?) - OpiWiki is collaboratively edited. OpiWiki's subject matter is defined. OpiWiki's layout is beautiful, especially with the updates I'm working on at this very moment, coming in a few days.
reddit is mainly a content sharing, social networking, comment, and entertainment website. However, some reddit subforums are typical discussion forums.
So, OpiWiki and reddit are two very different sites, having one common element: both sites allow discussion. I think a much more interesting question would be: Why should I choose OpiWiki over reddit looking for a website to discuss things?
I'd say you should choose OpiWiki when you're aiming for a serious discussion, or if you move more effectively in well-structured environments. We have certain guidelines to make our discussions meaningful and substantive. While reddit is a comment site by default, OpiWiki is The Encyclopedia of Opinions - every opinion has its own page and we treat every opinion just like a Wikipedia article is treated, means our opinions are emphasized, and they're here to stay. We also have a very cool system of karma, community accordance, and other user points. reddit's main advantage is obviously its popularity. But, as soon as we're out of beta (in a few months), we'll be progressively closing the gap.