This is basically a follow up for this message on Twitter: https://twitter.com/ZenphotoCMS/status/598549642553491456
140 letters is a bit too short to reply, so I prefer to do it here.

On my Zenphoto Theme, Paradigm (http://www.france-in-photos.com/pages/zenphoto-theme), I changed the way tag links are generated, basically removing the "nofollow" on these links.

Doing this allows search engines to index this kind of page: http://www.france-in-photos.com/page/search/tags/sunset

The idea is to provide search engines with numerous pages highlighting specific keywords. Those keywords can be different from the ones used elsewhere in the website: in gallery titles for instance. It is a quick way to provide more keyword coverage. It is easily done if you manage your tags in Lightroom and export them to Zenphoto.
I have to say this not work very well on my website: tag pages are not optimized enough to be really effective. The major landing pages for search engines are the albums, then the homepage and individual images. A tag description field (like in WordPress, but in rich text) could make these pages more effective.

The second aim is to give alternative paths to find image pages. In large galleries (1000+ images), the tag pages act as "thematic hubs" and distribute links in an alternative way. This may reduce the number of steps search engines have to take to discover your whole structure, though it also adds lots of new pages for them.
On my website, the gain is probably one depth level.
Tag clouds are also effective for this too, though on large galleries, they require lots of calculations and slow down page loading (higher time to first byte). This is why I limited the use of the tag cloud to search pages in the latest version of my theme.

We can remove the `rel="nofollow" or make it an option parameter. I think it was added because it was once a recommendation to do so. But that may be outdated by now.

We can consider a tag description field like on categories as well but I am not sure if that doesn't clutter the tag interface unnecessarily. You usually have a lot more tags than categories.

Did some quick research and it seems the nofollow on tags does not really make sense at all. I will remove them.

Removing the nofollow from the links and tag cloud makes sense: if people choose to have tags on their image pages, it is logical to have the pages indexed.

Having a rich text description field for tags would allow users to set up alternative navigations to their images: additional ways of sorting them (apart from albums).
It will probably be difficult to manage them if you import tags from your images metadata, but the added text will help them perform better.
WordPress has a nice interface for managing tags, with a tag cloud, quick add and tag manager. However, it lacks a list of linked elements in the tag management page itself.

I am familiar with WordPress. We will put the idea of tag descriptions on our list and see if we can fit that into our interface. I find ours generally better than WP's if you have hundreds of tags and the interface will get better with the planned next major release, too.

[i]Having a rich text description field for tags would allow users to set up alternative navigations to their images[/i]
You could basically create a dynamic album for each tag to do similar. Manually that is of course a lot of work but it can be code as well.