Things are not necessarily in order of importance. Often introduces the strongest point in an argument, the last point of a message, or the last person to be formally introduced. Sometimes spitefully or facetiously reversed (e.g. "Last and definitely the least").
A problem or obstacle that may be trivial in itself, but causes cataclysmic failure because it pushes the total array of problems or obstacles to an intolerable level. Also referred to as the Straw that broke the camel's back, after the original proverb: a straw by itself has an insignificant weight, but enough of them together can be a crushing weight. Another variant is the "drop that broke the dam."
Said of a person that is lacking intellect and/or sanity, even if they may appear at first to possess full mental faculties. Like "two bricks short of a load", there are endless variations, based around the metaphor of a machine or a system that is not operating as it should. ("His elevator doesn't stop at all floors.")