Open all Library Documents in Read-only Mode by Default
It's critical to have an easy ability to configure libraries to have documents open in read-only mode by default. The SP Online defaults make it very easy for content to be changed by mistake.
I have clients where they share documents for colleagues to just view, the docs open in edit mode by default, and changes are made by mistake because of auto-save. The clients are very frustrated with SharePoint Online as a result.
It should not be assumed that everyone with edit/contribute access to a library needs to edit a document they receive via a link or when they open it directly in a library to read it.
It is not a viable option to have all users set each document to "Always Open Read-Only" via the document protection in Word.
What's needed is a simple option/flag for a document library (or site) that will open all document in read-only mode by default, providing an option to switch it to edit mode.