This is a question/comment on something I've just run into with the new Gallery system -- it might be something to be fixed in future updates. I could be wrong in my assessment of the situation, so please let me know if I'm just barking up the wrong tree and missed something obvious.
I really like the new gallery system in 3.5.x. The way it previously worked (one gallery allowed per page unless you set up some individual pages for galleries and then called them in by ID) was one of the only things I didn't like about WordPress. It was not intuitive for non-technical web design clients. The new system is visual, easy and logical -- sometimes you want to list the same media file in a gallery on two pages, or even in two galleries on the same page, this makes that possible.
However, I just noticed while updating a client's website which had a photo gallery, that media files attached to the post while using the previous gallery system still show up as the gallery, even if I change the ID numbers in the list of images to be shown -- the gallery fails to update. I can change the number of columns, etc. effectively, but the images shown are still based on the media files attached to the page.
It's not too big a deal to create a new duplicate page and re-insert the gallery using the new system to ensure a clean setup, but it would be great to have the change to 3.5.x replace existing galleries with the new system and not rely on attachments to a post/page.