Complex passwords are often enforced by an organization's IT department to ensure that the user accounts of employees aren't externally compromised. They're also enforced in situations where a user is trusted to have access that could be damaging in the wrong hands.
In this case, neither situation applies. Stack Exchange doesn't store any private data, and regular user accounts can't do any damage. The only case where these password requirements might make sense is if the Stack Exchange systems were compromised, everyone's password hashes were exposed, they could be easily cracked, and users used the passwords elsewhere. No other scenario makes sense - Stack Exchange can throttle brute force attacks; if the login system is compromised password strength is useless; and even if the user's password is compromised, the damage it can do on Stack Exchange is quite limited. So in all cases, Stack Exchange is not affected by the strength of the passwords of their users.
On the flip side, the strong password requirements will likely cause many people to avoid using the service, and stick with their Google or Facebook accounts, both of which have far fewer restrictions on what the password can be.
This is an issue of degree - how secure can you force people to be before they either avoid your service or sabotage your attempts to "secure" them. For example, someone may have a password they would like to use, but it might not have 8 unique characters, so instead they use "Pas$word". Or they make up a password but know that they won't remember it, so they write it down somewhere that anyone that's near them could see it.
To provide some perspective, here's an examination of the password restrictions that the other OpenID providers prominently featured on the Stack Exchange login pages have:
Google: 8 character minimum, must also not be a "weak" password, though the exact mechanism they use to determine a "weak password" is obscure. ("abcdefgh" and "abcdefg1" are weak, while "Abcdefgh" is sufficiently strong). They do provide a page with tips for a stronger password.
Yahoo: 6 character minimum. There is a graphic similar to Google's that shows how strong your password is, but nothing prevents the creation of an account with a weak password.
MyOpenID: 1 character minimum, no other restrictions. Similar to Yahoo, there's a graphic that shows how strong the password is (source code), but nothing prevents the creation of an account with a weak (or even 1 character) password.
AOL: 6 character minimum, must also not be "too insecure". There is a password strength meter (source code), and it seems that any password that gets a score lower than 34 is considered "too insecure".
Facebook: 6 character minimum.
Compared with these, the restrictions on the Stack Exchange OpenID password are far stronger. Unnecessarily so, in my opinion, given that the largest pure OpenID provider, MyOpenID, has absolutely no password restrictions at all.