When a tweet contains a #hashtag, it would be useful if Tweetdeck had an option to display the meaning of this tag. This can be done using the api (http://api.tagdef.com) of www.tagdef.com. Tagdef suports multiple definitions for a tag, and the api can either return the top definition, or a sorted list of definitions. This can be useful as tags sometimes have more than one valid definition.
full disclosure: Tagdef.com is my website, but I think other users of Tweetdeck also might find it useful.