1. Analogy: Ehrenreich compares Wal-Mart's expansion to an alien invasion this is shown just through the title, "Wal-Martian Invasion." This analogy is very useful because it very clearly shows the authors opinion right off the bat. It also helps her to level with people in her audience who have similar feelings about the rapid growth of Wal-Mart. Furthermore, the analogy could alarm neutral readers and make them more anti-Wal-Mart.
2. Imagery: "Employees have been locked into stores overnight; unable to get help even in medical emergencies." This image is a very vivid example of the entire point of the article. By giving an extreme example of Wal-Martian cruelty Ehrenreich is able to make the audience more anti-Wal-Mart.
3. Overstatement: "My thumbnail calculation suggests that by the year 4004, every square inch of the United States will be covered in super centers." Obviously Ehrenreich doesn't expect Wal-Mart to engulf the world, but her comment still reflect the alarming growth of Wal-Mart.