Allow for a transparent Manual vs. Automatic Calculation setting by workbook vs. a global setting.
When you have one calculation-heavy workbook and distribute it to other people, it would be best if you could turn off calculation for only that workbook, without affecting the other workbooks these users are working on. It is confusing and outright dangerous in potential consequences that suddenly their workbooks do not calculate anymore. On the other hand, if the calculation is on, their whole excel freezes for a considerable amount of time every few keystrokes.
Suggestion: Allow for a transparent Manual vs. Automatic Calculation setting by workbook vs. a global setting.
There should be a popup when you open the workbook if the calculation has been turned off by the previous user of the document, giving you the choice to accept this (applicable only to the workbook) or activate the calculation again.
good description of the issue and a solution I'd be happy with.