In the last time I had the feeling that more and more new users came put with names like “user1234”. In this cases I suggest them to change the name, but I wonder if we can prevent this and enforce new users to define another name …
We've had a number of questions regarding the enforcement of "policy" about site usage. And, it most cases, such enforcement is really not possible, nor warranted.
We have many users who literally come-and-go after having received help. To them, userXXXX as a username is sufficient, and they don't really care, nor should we. It still represents an identifiable, unique username. It's probably better than requesting a change and the user adopting the vague username Anonymous:
Users should have the freedom to choose their names and how it's displayed, just like they can their avatars. Private information (which includes the user's "real name") is considered private until otherwise designated by the user.