Some pool operators use the standard bitcoin to decide - like me

Other pools (like eligius) black list addresses coz they think gambling sites are a ddos on bitcoin
and add hidden rules about what transactions they will ignore ...
and had agreements with companies like MtGox to accept their transaction through a special interface ...
(and send their miners blocks with empty transactions to speed up their slow pool software ...)
who knows what they will black list next ...

If you mine to a pool, consider if they have hidden agendas and hide what they do, or are open and use the standard bitcoin rules