XnView Software

It would be nice if we could see the list of categories in a sidebar on the full-screened images to add categories to images that we're viewing in the large view.

Yes, that's what I was asking when I read your post...
In the view mode, there's no way to write IPTC/XMP tags...A small panel could be a solution, but maybe I am still with picasa bad habits

And what are the main differences between categories and tags, as we can see the tags in the "other" categories ?

I like to add captions / tags to my image, often while I'm in image view mode but for some reason, the Edit IPTC/XMP button is not available on the single image view toolbar.

As a bonus, it would be great to be able to have the IPTC caption visible when flicking through images. I've discovered that both Facebook and Flickr use the IPTC caption value as their caption when images are uploaded so this is a very useful field to populate. I much prefer editing captions locally than on social media websites so I'm happy I can do this on XNView MP, just want to make it a bit easier to read what I've entered.

fractl wrote:As a bonus, it would be great to be able to have the IPTC caption visible when flicking through images.

I belive this can be done If You mean fullscreen view. Have you tried Tools>> Settings>> View>> "Info Tab". Try to Add "IPTC:caption" under "information displayed" Box.