The primary changes in TOM are the addition of multi-inheritance, tuples as a first-class part of the language, cleaner syntax, free of the C requirements for header files and pre-compiler commands, and the ability to use categories (the re-use mechanism) to include anything.
It is this later ability that represents "the whole idea". Unlike Objective C's categories that allowed only new methods to be built onto existing classes, TOM allows you to add class and instance variables, new methods, even new superclasses. This results in the redefinition of "class" as "a class is defined by its main definition and any extensions", these extensions have become a first-class citizen of the language.