The forums module has indeed been a good feature of Seditio and Cotonti, but in my opinion it's lacking some basic features that you get out of the box with other forums systems nowadays.
Things like:
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Subscribe to topic. Notify with instant notice, daily digest, via PM or email.
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Icon in topics row showing to yourself if you already posted in a topic previously.
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Nested quotes.
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Reputation system allowing users to give a +1/like on a post. Bit like Trustmaster's 'Thanks' plugin, but show this per post so others can see the value of a post.
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Option to quickly share a topic via social media and email.
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Feature for users to rate a topic.
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Filter threads on last so many days, rating, number of replies, number of views.
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Feature for notifications when somebody quotes one of your posts, or replies to one.
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Ability to use coloured usergroups. Useful for community websites with lots of different ranks/groups.
Just throwing in some ideas here.