I think its a good idea to add minimum stat for dps/tank u need to take cap.
For example, if u need 450 def, and now u have only 400 - there a red number "-50" near the def (or some mix).
For hit rating also a good idea to add raid buffs (shaman or some buffs) to see "minimum" cap u need and how much u need more - like "-3.29%".

I would love to see this for all classes and specs, that would be most helpful.

For Hit I was thinking about adding a new "Cap" entry, which will then be either -X.Y % or +X.Y %.

For Crit Immunity there is already a percentage value called "Crit Immunity" for which you need to reach 100% to be crit immune. Personally I think that is enough information and quite self-explaining.