I like that you can create folders of tags so it is easy to go back in time if needed or add new ones when they are created! It's very user friendly - just drag & drop them into their correct place for each post/page etc., no training required at all (which was one reason why i chose this over other systems).
If there were any additional features which would be useful then maybe more people will use tagboxes? We have been using TAGBOXs since 2013 but we don't manage our posts as well now because not many users know how good an asset tagging makes things look online compared with what happens without tagged articles being published; however once someone has used these tools before its much easier than trying again elsewhere due mainly doozey interface design by Taggist itself!!