Religious intolerance has its roots in exclusivity - believing that only one religion can offer salvation and that the only alternative to salvation is hell. The exclusivist can only see other religions as evil i.e. leading to hell, and thus intolerable. While the Buddha clearly taught that Nirvana can be realised only by practising the Noble Eightfold Path, he did not believe that the only alternative to this goal was hell. One can take rebirth in any of the six realms of existence. Just as importantly, where one is reborn is conditioned by one's behaviour not by which god one has faith in. Any good person can have a good rebirth no matter what their beliefs and consequently the Buddhist is able to acknowledge and appreciate the good in other religions.
Other beliefs that make Buddhism tolerant are the ideas of impermanence and rebirth. The first means that even if a person does go to hell it won't be forever and the second means that if one does not spiritually progress in this life one will always have the opportunity to do so in the next. The combination of all these ideas means that Buddhism judges other religions by how they advise their adherents to act rather than by what they tell them to believe. As most higher religions promote values like honesty, kindness, generosity, courage and integrity, Buddhism sees them not as dangerous competitors but as allies in man's quest for liberation.