Plotplotplotplotplotplotplot. More plot. It must be logical, it must be complete (see rant on review of Sister for what happens when a plot is not complete), it need not be realistic if the genre is appropriate (i.e. I know that every Clive Cussler book involves locations on at least 4 continents) but it helps.
Really stonking good characters. The protagonist doesn’t have to be reliable, but I can’t deal with murderous/adulterous/useless ones. I want to want things to end well for them. There’s got to be a humorous side-kick (e.g. Grandma Mazur in the Stephanie Plum series), and preferably a frustrating mother/spouse/distant relative who cramps the protagonist’s style.
Obviously I wouldn’t pass something unsuited to a friend as a must-read (I don’t think I’ll ever be recommending DanBrownesque thrillers – can we just make this an adjective already? – to TNLRC) but it’s going to have to have at least one of the above and no must-read can be seriously lacking one or the other.