TL;DR: If your language has the right tool, your classes should be either abstract or final.

Problems

Subclassification for Code Reuse
Classes with just one concrete subclass
Liskov Substitution Violation
Yo-Yo Problem

Solutions

Declare all your leaf classes as final and the rest of them abstract.

Context

Managing hierarchies and composition is the main task of a good software designer.

Keeping hierarchies healthy is crucial to favor cohesion and avoid coupling.

Sample Code

Wrong

public class Vehicle
{
  // class is not a leaf. Therefore it should be abstract 

  //an abstract method that only declares, but does not define the start 
  //functionality because each vehicle uses a different starting mechanism
  abstract void start();
}

public class Car extends Vehicle
{
  // class is leaf. Therefore it should be final
}

public class Motorcycle extends Vehicle
{
  // class is leaf. Therefore it should be final
}

Right

abstract public class Vehicle
{
  // class is not a leaf. Therefore it is abstract

  //an abstract method that only declares, but does not define the start 
  //functionality because each vehicle uses a different starting mechanism
  abstract void start();
}

final public class Car extends Vehicle
{
  // class is leaf. Therefore it is final
}

final public class Motorcycle extends Vehicle
{
  // class is leaf. Therefore it is final
}