Hi Nicolas,
When we were originally developing Review Board, we were strongly considering tags as a way of organizing review requests. We played around with it and decided against it, favoring the groups method instead.
We try to keep workflows pretty sane and fast, and we felt that tags would just complicate things, based on our past experience as users and developers of tag-based systems.
Tags have the problems of lack of standardization. Should the tag for this review request be "UI", "User Interface", "Linux UI", "GTK", or "GTK+"? All might be valid, but different people are going to have different tags they think of when they go to tag their review request.
As a reviewer who is then trying to keep an eye on all tags that have to do with the UI, they'd have to periodically go through all the known tags (these add up!) and add each new one they find to their filter, or they'd easily miss something.
It's additional work for all involved. That's why we went with groups. There's a list of known groups. They must be explicitly created. They have members. You can't typo them and get a new group. And yet, organization-wise, they can be just as useful as tags.
So we won't be adding tag support to Review Board.
Now, if tags are something you really want, I'd recommend writing an extension. It might at the moment test the limits of what extensions can do, but it'd be a great way of seeing what needs to be added to our extension framework. You could (in 1.8, not yet), cleanly introduce fields on a review request for specifying tags, add database models to link things, add new API for tags, add a page for showing all tags. That way it's something you guys could use, maybe even make available, without having to modify Review Board itself.
That would require some in-progress work we're doing in 1.8 to make it possible to add custom fields to a review request. I'm actively working on the change for this right now, and it will land in master at some point before long.
Christian