Updating a friend's site today, which uses K2, reminded me of a couple of features that would be terrific.
The ability to edit captions of image gallery photos through the editing interface.I know how to use the labels.txt file, but the great advantage of Joomla is it's a tool which allows non-technical users to admin their own site without having to learn about FTP. There seem to be hundreds of lightbox-style image gallery plugins for Joomla, but I don't know of one of them that lets the admin edit the captions through the management interface. If K2 could add that feature to manage the SIGpro images, that would really make it stand out even more.
With that same image gallery, he wanted to add one photo to the 25 that were already up. This isn't something you can do through the GUI. You have to reupload the entire gallery. This required me to FTP the new image to the media directory and manually update the labels.txt file. It would be more elegant to allow the adding, editing, removing of single images within a gallery set.
During this it occurred to me that you are limited to one gallery and one video per article, but multiple downloads. I can see situations were I would definitely find value in posting multiple videos per article, and maybe even multiple galleries. I know that you can use the article plugin code to do this, but it would be cool to see the built-in gallery and video tools operate more like the Downloads, with multiple entries allowed.