The other major difference between a class action and mass action is whether the claims are similar enough, either factually and/or legally, to be able to prosecute them as a whole. Class actions can only be brought if they meet all the requirements of Federal Rule of Civil Procedure 23 (or a similar state rule, such as Indiana Rule of Trial Procedure 23), whereas mass actions are a default when all such requirements cannot be met.
There are many procedural rules set in place for determining when a class action is appropriate, but ultimately the idea behind each of them is to make sure that the person who is the class representative can adequately represent the interests of his or her fellow class members. This was touched upon above, but it also is important for the other two major requirements for a class to be certified — commonality and typicality. The idea is that a class representative could not adequately represent another absent person if they do not pose questions of law and fact common to the class, and if the named parties claims or defenses are not typical of the class.