2. The Fourth Doctor's personalities
The character of the Fourth Doctor himself is so incredibly distinctive. In some ways, he's very much recognizably the Doctor we had seen before in his previous three incarnations. But at the same time, there was something brand new. Something perhaps more alien than we had ever seen before.
The funny thing about the Fourth Doctor is that, when watching his seasons in order, it almost feels like there are three distinct versions of the character. The first is alien, mysterious, and generally takes threats seriously. Possibly because he's often up against extremely dangerous threats such as an alien seed that could wipe out all life on planet Earth; an old foe risking Gallifrey itself to extend his own life, or even a powerful god-like being dedicated to wiping out all life in the universe.
The second personality is the one that prefers to hide his intelligence. Quite often, this Doctor will play the fool, laughing at his own expense while learning as much as he can about his enemies. However, when the stakes are high - as they often are - that's when he starts to act deadly serious.
The last version of Four is introduced in his final season. This one is generally more melancholy. He's back to taking things more seriously, but in a different way than he used to. Almost like he knows that the end is coming.
Each of these versions of the character always felt compelling to watch. No matter how good or bad the story was, we knew we'd be entertained at least by the character of the Doctor, who always felt real and fleshed out.
Of course, there's one big reason for that, and that's the number one reason why the Fourth Doctor is still so loved to this day...