5. Strong, distinctive companions
Watching Tom Baker's seasons in order, I was amazed at how many truly great companions were created during this era. Sarah Jane Smith, Harry Sullivan, Leela, K9, both Romanas. All of these characters are distinctive and memorable and feel fully fleshed-out.
They also contrast against each other brilliantly. While Sarah Jane was very much a modern woman, Harry was extremely old-fashioned. While Leela was savage but intuitive, K9 was sophisticated but rigidly logical. Even the two Romanas stand out from each other, in their own ways. Mary Tamm's incarnation is noticeably more glamorous than Lalla Ward's, but both provide great takes on the Time Lady.
Each of these companions stood out on their own terms and worked brilliantly against Tom Baker's Doctor. And he would never have the same relationship twice. With Sarah Jane, he could be a friend. With Leela, a mentor. With Romana, he could even play the fool and let her be the clever one. Each companion brought out something different from the Doctor, and it worked brilliantly.
For most of the Fourth Doctor's era, he had no more than two companions at a time. This helped to give each companion plenty of time to develop and stand out on their own terms. This started to change in Tom Baker's final season, and by his final story he suddenly found himself with three companions, which might've been too many.
But for the majority of his run, the companions - while not given quite as much development as those in the New Series - were strong and distinctive enough to truly stand out, and therefore, helped to make Tom Baker's era even more special.