First off, let me just tell you that Gallery is one of my favorite pieces of free software I have ever used. This is actually one freeware that I would actually donate to!
With that said, I think there should be two changes with Gallery 2.x.
1) Intergrate it better with PHP-Nuke. For example, instead of having a seperate Admin account, make it use the PHP-Nuke ones.
2) Description vs. caption. I believe you should have a description AND a caption for each picture. The caption would be on the thumbnail screen, and the description on the full screen and slideshow. I like to have lengthy descriptions, but it makes the thumbnail screen look cluttered.
Thanks for you time. I'm sorry if these have already been stated, but I just wanted to input my opinion. Keep up the good work!