I think skill level, bit size/type and general descriptive tags for each project. Material doesn't necessarily matter since it can be swapped around.
Carving time would be nice, especially if a range was given (0-30 min, 30-60 min, 1-2 hr, etc.)
It may not hurt to include which tool (300W spindle, DW611, etc.) was used, as well as which program was used. No need to be looking at projects that were made in V-Carve if I only have Easel.