I might be confused, but in this case I don't think so. I get what you're saying, but what makes a book review a different type of content than a post or a recipe or a store location?
It seems the fields of data (or images/files I guess) and not the labelling is what makes them different.
Well, for "practical" purposes, you can change the fields of the "post" post_type on a per post basis - creating any content type you want. If that's true, it seems that grouping, manipulating and displaying these "content types" is what most are after. I believe that grouping can occur right now through labelling/categorization.
As a result, I would add a nuance to your answer to my question:
What can you do with custom post types that you can't do with categories/taxonomies?
* Create content.
* Label it with taxonomies.
* Add an excerpt.
* Set a published status.
* Add custom metadata.
Of course, technically, you're right. However, I can label, excerpt, change status and add metadata to a "post" post_type and by it's categorization/labelling, separate it from other categories of posts for display, manipulation, etc. Since that is the end goal - at least in my implementation, I still don't see what additional benefit I get from custom post types that have "native" custom fields associated with them rather than those added to a "post" (except elimination of the unneeded fields associated with all "posts").
I'm not trying to be difficult. I too want to see content_type-aware plugins and native functions that make use of this positive additional layer of abstraction. When the wp_postmeta table becomes just contentmeta or multiple different tables, I can see WP's CMS street cred growing. I'm glad you're writing a post so plugin authors can "see" the future. I just can't wait for 3.0 and don't see that I'm missing out on much in functionality by not waiting.
In fact unless custom content types have a different database structure (e.g. multiple wp_content tables with only relevant fields), I don't see the practical benefit at all. Yes, adding custom "write panels" that allow new content types with associated custom fields will be nice. Sure, adding the ability to access those different content types with native functions will be helpful too. I just think you can do all this right now with the hackish solution of Flutter and the post post_type.
If I'm wrong, I'd like to know, so I can add the correct information to my planning. I'm normally a measure twice, cut once type of guy.