His good looks and easy, effortless acting.
He played bad guys and good guys and grey ones with equal aplomb.
His movies were usually watchable, and sometimes excellent. Mere Apne, Achanak, Amar Akbar Anthony, Inkaar, Kudrat are some that I can remember readily. Lahu ke Do Rang too.
His Imtihan (which had the inspirational song Ruk Jaana nahin tu kahin haar ke) was a remake of To Sir With Love, a film about the beautiful teacher-student relationship, and how a single teacher can transform the attitude of students. Later, a Robin Williams film called Dead Poets' Society would do a nice film on a similar theme (that's also my favourite).
He is also remembered for going away to live with Osho in his Oregon ranch when he was a successful actor. His role in Qurbani with Feroz Khan was also quite memorable.