Question: I'm using the All in One SEO plugin for my site. The extra fields and options it supplies seem like they could be handled via a theme, possibly negating the use for a plugin.

Should SEO considerations be handled by a theme, rather than delegating the work to a separate plugin? It could reduce maintenance issues, takes up less space, and be more integrated.

Answer: 
Custom types can be leverage for anything that can be repeatable. So as you said a gallery or portfolio would work very nicely. I've already used them on a few different projects: 

* Recipes
* Employee Roster
* Callouts for the homepage
* rotating sliders.

Other ideas are:

* products for a commerce site—The custom taxonomies would make for some powerful option for each product.
* For custom sidebar content. Y
* An Ad manager.

The possibilities are really endless. Anything that can be seen as a collection is really what this is for.
I'm considering using it for a client who runs a tree nursery. This would allow me to logically separate blog-style news updates, pages, and trees (with per-tree galleries) with custom styles for each post type. It'll also be nice to have type-specific taxonomies for categorizing the trees.