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Quotations style? But where do you draw the line? At links, quotations, italics,... I do think at some point it will collide with your forum-goals.
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What are the most common things in forums? Normal threads about random topics, tutorials, replies to other people who commented, quotes from other sites and gigantic snippets of code. Lets make it so these are easy.

In my opinion, quote and code blocks are essential. Anything else is a nice or not-so-nice extra. Lists could be useful, but not necessary. Beyond that, the only useful things I see missing are two features only for admins and moderators: the ins and del elements.