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Formatting of dates in any format - no having to create fields to format dates, because Salesforce completely sucks at that.
Formatting currency in any format - bonus points for full excel style formatting where you can format for positive, negative and null values (I’ve never seen that though).
Formatting percentages - with or without decimals, and handling null (eg display as 0%).
Formatting numbers - with or without decimals, and handling nulls.
Conditionally displaying text - easy with Word, I just use Word formulas, but if your Doc Gen solution is not on Word, it needs to be easy and robust and allow for many conditional elements in the document.
Conditionally displaying paragraphs, sections, bulleted list points etc. Your conditional text is not always inline text, it could be whole paragraphs or even pages.
Displaying Images from merge fields - we are not in 1999 anymore - this must be a given. Including Re-sizing the images to fit the desired area.
Displaying HTML from within Salesforce - yes this is hard because Salesforce Rich Text fields suck badly, but you have to be able to do this, at least basically.