I like this plugin a lot and i wanted to add in some suggestions for fuure development.
1. A lot of the tags on my site are in lower case. When I see them in the tag archive title, they are also lowercase, which doesn't look as good as uppercase. I suggest you add a checkbox for people who want all tag and category titles to be uppercase.
2. One thing that might be useful is the ability to add _yoast_wpseo_metadesc and _yoast_wpseo_title custom fields from the dropdown menu in the custom metadata box in the write panel. A lot of time, I want to go back to older posts or to new posts where I have left out a custom title or description and add one WITHOUT updating the post. If I could just add the custom field, then I could do this without triggering all of the server CPU stress associated with the post update.
3. In the same vein, it would be terrific to have a tab in the titles menu (page=wpseo_titles) which allowed one to put in specific titles for categories and tags (and maybe even pages and posts). That way, a new plugin user could see all the tags or categories in one place and change the titles and/or meta descriptions. From a workflow perspective, that would be super for anyone just installing the plugin.
4. I'm not sure if you are focused on images but I don't think WordPress adds the alt descriptor to images automatically. If your plugin could do that by copying the image title over, that would be a huge bonus for image SEO.
Anyway, that's it for now. Super plugin and look forward to the next iterations.