Answer:
The main differences between the two arises from practice and study. In Mahayana Buddhism the studying of sutras plays a crucial role in understanding reality. In Tibetan Buddhism, more emphasis is placed on studying the texts of the Prasangika-Madyamaka school (supposedly derived from Nagarjuna's teachings). Also Tibetan Buddhists constantly engage in philosophical debates on Buddhist principles to cultivate their wisdom. However it must be noted that the Mahayana sutras and Prasangika-Madyamaka schools both study the emptiness philosophy of Nagarjuna so in essence they should be the same (differences only arise in the conceptual dimension). Also, the Mahayana school has the Avatamsaka Sutra which is not found in any other Buddhist school (it is the Buddhist philosophy of interpenetration, interconnectivity and recursion and is considered to be the most advanced metaphysics in the world). The most famous book in Tibetan Buddhism is the Tibetan Book of the Dead which deals with the consciousness states.
Another difference is that Tibetan Buddhists will often practice tantra which is not followed in Mahayana where the highest practices are the perfections.