Pieces that have crit are ideal to reforge for mastery - while crit is a decent stat, mastery offers much more. If you have no mana issues, spirit is also something you can safely reforge. However, mana has become more of an issue since 4.0.1, so do be cautious. Mastery won't help you if you're OOM.
TLDR: Crit is ideal to reforge, spirit can also work.