Assorted Puzzles (WORK IN PROGRESS!)

A note: none of the puzzles here will be of much help if you're working on the larger Cryptograms puzzle.

Visual Reasoning Puzzle

Which of the following shapes doesn't belong with the others?

Notes: There are many possible answers, but the one I had in mind (a) relies on a rule that can be summed up in one simple sentence, and (b) doesn't rely on any nitpicky properties like concavity, or whether they tile the plane, or whether I drew the sides perfectly in MS paint.

Credit: yours truly.