What are hashtags?
Users who use twitter may know a lot about #hashtags, it has recently been added as a feature in Facebook as well. "Hashtags" are content tags, that can be clicked upon to view more content of similar nature. For example, if I were to add a hashtag #XenforoNews, it would turn into a hyperlink and can be clicked upon to see all content containing the same hashtag.
How is it useful?
Many communities are more than just a "community", they provide resources, articles of various kinds, news and information, products or even talk about everything under the sun. These forums always look for "organization" of content, most prefer using "tags" that can be added and auto-linked so that users can find similar content or it can be searched for easily.
Hashtags make it easier, by making it inline, you do not need to do anything or add any tags after the thread/content is created, and hashtags can be used in threads, replies (posts), member profile updates etc making it easier to organize content of the same type. This feature can be useful to communities/forums that provide news/information or even general communities for better organization and a bit more #fun.
Hashtagging is #useless
Many people may have this opinion, since some twitter users did make the hashtag look that way, but we cannot hate an idea just due to a certain few can we? Imagine, XenForo having its own hashtag system, similar to the user @ tag, it's going to make it more engaging and social, better to organize content, better to search for relevant content etc. And I'm pretty sure this will please everyone looking for a content tagging system. Maybe an official sidebar block to show the hashtag cloud? Wouldn't it be sweet?
Others may have even better ideas than me to make use of the hashtag, please feel free to share your opinions and if you feel this feature should make it into #xenForo core.
Edit: More Ideas (and future ideas) to improve upon base suggestion:
Users can "watch" hashtags and be alerted for new content containing that hashtag
Once you click on a hashtag, it takes you to a search like page where all content with that hashtag is displayed. You have an option to "watch" the hashtag, and be notified via Alerts/E-Mail when new content is created with that "hashtag", alert options can be set to alert for "threads", alert for "posts" containing hashtag, or alert for both.