In the first few months after beginning to systematically unit testing components, most developers I have encountered will have qualms about modifying the component under test (CUT) so that it is easier to test. In a typical scenario, the component under test will need to expose class members for testing purposes. However, exposing members goes against the information hiding principle. Thus, an otherwise well-intentioned developer may go to great lengths to perform the tests without exposing new members, or alternatively may expose said members but feeling guilty about compromising the information hiding principle.
It takes some experience to realize that exposing such members is a low price to pay for increased testability. At some later stage, you may even begin considering tests as an important force driving the design of your components.
Testable components expose more of their internal state to the outside world. They also tend to be more modular. For example, if a CUT requires a database connection, then the developer might modify it so that it admits DAO and then inject a mock of the DAO using a mock database, allowing the CUT to be tested independently of the database.
Persistence, concurrency, distribution and transaction support are accepted to be wide-reaching and fundamental aspects of a component. For instance, a component designed for persistence may differ substantially from the same component without persistence support. In a similar vein, a component designed with testability in mind will show different characteristics than its less testable counterpart. In particular and as mentioned above, testable components tend to be much more modular. It follows that modularity is a corollary of testability.
It should be noted that, it takes significant effort to develop and maintain test code. In the logback project, we spend about 30% of our time on documentation, at least 50% maintaining tests, with only the remaining 20% of time spent on the actual code. Writing good test code requires real skill.