The characteristics that help make an informal letter friendlier and more conversational in tone are the following:
Uses contractions.
Uses slang and colloquial language.
Reveals the writer's personality.
Uses underlined words for emphasis.
Uses expressive punctuation.
Uses the reader's name in the body of the letter.
The other features, "explains why it is time to end the letter" and "including an apology, explaining why the writer hasn't written in so long", belong to a more formal style of writing.
