8. The costume
This may seem simple, but a Doctor's costume is undeniably important. Honestly, the Sixth Doctor's costume might've been a key reason why he wasn't more successful. Certainly, the Sixth Doctor has seen greater success on audio than on television.
It's funny, but arguably what helps the Fourth Doctor's costume is how simple it is. A long brown coat with a matching hat. Trousers and shoes, waistcoat optional. In some ways, it's surprisingly straightforward.
Of course, what really completes the costume is the scarf. Incredibly long and full of a range of colors, the Fourth Doctor's scarf is the first thing that everyone remembers. The fact that the rest of the outfit is so simple not only helps the scarf to stand out more but also helps to ground it.
It's a look that's so simple, that as soon as you see it, you know it's the Fourth Doctor. More than that, for the non-fans, as soon as they see the costume, they'll know it's "Doctor Who". There might be people in the UK who have never seen an episode, but if they were asked to think of a Doctor, they'd likely either say "David Tennant" or "the one with the scarf". It's that iconic.