The Single Responsibility Principle states that a class should do one and only one thing. Some cases are pretty clear cut. Others, though, are difficult because what looks like "one thing" when viewed at a given level of abstraction may be multiple things when viewed at a lower level. I also fear that if the Single Responsibility Principle is honored at the lower levels, excessively decoupled, verbose ravioli code, where more lines are spent creating tiny classes for everything and plumbing information around than actually solving the problem at hand, can result.

How would you describe what "one thing" means? What are some concrete signs that a class really does more than "one thing"?