One thing that burned our test users with Impress is that when images are dragged-and-dropped in, they are linked, not embedded (PowerPoint defaults to embedding). This caused one of our users to take an image-heavy presentation they had carefully assembled and have no images show up on the presentation machine.
When you add images using the menu in Impress (testing with 3.0), you can choose to link or embed, but when dragging-and-dropping, which is what our users were used to, the default is link. This cannot be changed as far as I could determine (see bug 15369).
Impress also converts embedded images to PNG, which can really inflate their size compared to JPG.