The most unique characteristic of the Mahayana is the Bodhisattva concept. In many different schools of Mahayana, practioners take Bodhisattva vows. There are various forms of these vows, but the essential point is that one vows "not to enter final nirvana until all beings are liberated."
It may be a surprise to some, but there is a bodhisatta path in Theravada. (Pedantic linguistic note: whereas, scholar-types tell us that contrary to appearances, bodhisatta is not the pali equivalent of sanskrit bodhisattva, I've never seen that explained and they look like equivalents to me.)
However, the concept is quite different from the Mahayana formulation. In Theravada, a bodhisatta is one who is practising for Buddha-hood. A follower of the path can make a resolve to follow one of three destinies; to aim for arhantship, pacceka-buddhahood or full samma-sam-buddhahood. In actual practise, very few Theravadins take a bodhisatta vow. The road to buddha-hood is considered much longer and more onerous. Only someone motivated by a great compassion would forestall their own liberation for the hundreds of life-times required.
Perhaps that is the historical germ of the Mahayana idea; which certainly places a central emphasis on compassion. The concepts are quite different, but I think we can see a direct line of descent here.
Another important point is that, in Theravada, such a vow is not considered complete or binding until one makes it in the presence of a living Buddha, as Gotama did before Dipankara Buddha many aeons ago. Some say this is the meaning of the epithet "Bhagava" or blessed; that a Buddha was blessed in the distant past by another Buddha.