I'm currently a new DM using the Lost Mine of Phandelver Starter Set, and the party recently received the "Staff of Defense" magic item:
This slender, hollow staff is made of glass yet is as strong as oak. It weighs 3 pounds. You must be attuned to the staff to gain its benefits and cast its spells.
While holding the staff, you have a +1 bonus to your Armor Class.
The staff has 10 charges, which are used to fuel the spells within it. With the staff in hand, you can use your action to cast one of the following spells from the staff if the spell is on your class’s spell list: mage armor (1 charge) or shield (2 charges). No components are required.
The staff regains 1d6+4 expended charges each day at dawn. If you expend the staff’s last charge, roll a d20. On a 1, the staff shatters and is destroyed.
The Staff description specifically says "you can use your action" to cast the spell. However, the Shield spell has a casting time of "1 reaction, which you take when you are hit by an attack or targeted by the magic missile spell". And it seems that Shield is much more useful when you can cast it as a Reaction.
So, is this (1) a "specific beats general" case where the Staff of Defense requires an action to cast Shield even though the Shield spell itself usually doesn't? Or (2) since Shield is really all about being able to use a Reaction to try to block an attack, is the Staff's "you can use an action" just poorly-phrased shorthand for "using a spell's normal casting time"?
I'm inclined to think it's option (1) from a straightforward reading of the text, but since Shield just seems so much less useful if you need to use your whole Action on it, especially as it costs twice the number of charges per use as Mage Armor, I'm assuming I must be missing something.
Jeremy Crawford, lead rules developer for D&D, states that the staff allows Shield to be cast as an action when normally it would only be cast as a reaction. He also states that the DM may allow the staff to be used to cast it as a reaction if they wish.
"That staff does, indeed, let you cast shield as an action. A DM could also allow casting the spell as a reaction."
Taking from the errata on spell scrolls, it would seem that, as intended, you can use a Reaction to cast Shield from the staff:
Casting the spell by reading the scroll requires the spell’s normal casting time.
This seems to be the case even for the Staff of Defense, being in the Starter Set, which was released before the DMG errata was.