If someone wants to crack the passwords for some of your site's users, then knowing the password rules may help him reduce his list of "potential passwords" by pruning out the passwords that the server would not have accepted in the first place. However, if:
- the attacker can also register as a user (as is the case for many Web sites, where account creation is free);
- the "registration page" already lists the password requirements;
then the attacker can be assumed to already have that information. Hence, it does not harm security to recall them on the login page.
Anyway, maintaining password requirements secret would be difficult, since they have been shown to all users. A secret which is shared by more than 3 or 4 persons is no longer a secret. So it can be assumed that the attacker already knows your password requirements, and displaying them on the login page gives him no extra advantage.
There is a risk that displaying the password requirements on the login page may confuse some users, because such requirements are usually displayed on registration pages, not login pages. You will have to be careful in your wording and presentation. Another possible usability issue is if you change the password requirements at some point, for new passwords: some old, but still valid passwords may fail to match the new requirements, and, there again, the corresponding users may be confused by the mismatch. Yet, these are not security issues (except that confused users are, generally speaking, a potential security hazard).