Can I tag issues?
Not currently. It's something that we're seriously considering but really don't want to get wrong. In general, tagging is messy, inaccurate, and error-prone, and those are acceptable problems for things like bookmarking or photos, but bug and issue tracking is another story. When bugs slip through the cracks because of typos or people using different words for the same thing, that can create problems. We don't want that happening.
Currently, Sifter supports custom categories and milestones for each project, and we're exploring ways to add tagging via a controlled vocabulary. That way, you could create a set of tags for a given project and have multiple ways to organize it without worrying about typos or people using the wrong words. If you're interested in tagging, it would be really helpful if you drop us a line and let us know how it could help you with your workflow.