My view of Buddhism is heavily Mahayana and Vajrayana based, and I know little of Hinayana and the other 'vehicles'. In this context, emphasis is placed on one's teacher mainly because it is believed that it is very, very easy to understand things completely wrongly and to go off on a tangent that is harmful to oneself and others. The teacher is thus your guide, while the work of attaining enlightenment is still very much your own.
This of course is not to say that Buddhahood isn't possible without a teacher; it's just that it is rather more unlikely and difficult.