Tags are good for informal, temporary and overlapping grouping, typically when the scope is limited. For example:

daniel-current-working-set
UX-need-to-check
themes: usability, performance, security


Attributes, on the other hand ...
* Exist in all artifacts of the same type
* Have specific data types (string, enumeration, date, person, etc)
* Can be edited en mass in the project list artifacts view for a set of artifacts

For example, attributes are often used to represent state (draft, approved), target release, etc.

So you want to use attributes sparingly and put more thought into their data types and which enumerated values are appropriate.

You want to avoid, after a few years, having artifact types with large numbers attributes, most of them no longer used. Tags give you a way of doing the one-off groupings; attributes give you ways that are common and enduring.