other deductions are:
The similarity of letter headings (or titles) and newspaper headlines concise ,short, words, omissions, etc
The frequency with which writers identify themselves as having expertise or experience in the topic umder discussion (e.g. ‘I have served with the Federal Aviation Authority for 9 years, and I was amazed to read Mr. Cudden's critcism of the airline pilots' action…’)
The thematic and local concerns in the letters columns of particular papers : a comparison between a university and a local paper from the same city is an interesting exercise.
A range of words for describing tone, e.g. angry, guarded, timid, optimistic, pessimistie, detached. hopeful, indignant.