I really like Juicebox and would like even more to purchase Pro but the one main missing feature for me is more transitions.  I have been testing everything from HTML5 Slideshow Maker to Hi Slider to Juicebox trying to find HTML5 slideshow creation software that does everything I want it to do.

I could live without the rest of the features but I figure it can't hurt to mention them anyway:

Way more transitions - I want everything from basic fades to phenomenal 3d effects to pick from. (This one is a deal breaker for me and is what kept me from going Pro)

Multiple rows of thumbnails - I want to publish hundreds of photos to the gallery, a single row does not make the size of the gallery discoverable for end users

A grid option - I would really like to be able to display the entire gallery as an HTM5 grid with the slideshow at the top of the page. Any pictures clicked in the grid would display the picture in the slideshow and start playing the slideshow from that point in the grid.

Tags - I have seen some galleries that let you click a tag that will then display only the photos in the gallery that have that tag. Example - gallery contains pictures of horses, dogs, cats). At the top of the slideshow would be 4 tags (All,Horses,Dogs,Cats). Clicking a tag would then filter the slideshow (and grid) to only show the images with that tag.

Gallery Themes - I have tested some software that has some out of the box themes that lets you quickly make an attractive gallery. All of the shadowing, navigation buttons, etc are already created and optimized for you.  It is possible this is already a Pro feature, to date I have only tested Lite.