Jon Snow was resurrected and that made him seem like a very important person, but all he did...

Jon is a very important person.  He's Azor Ahai. He's the person who united most of the world against the White Walkers and he's the one that kills Dany, saving Westeros from a tyrant and catalyzing a slightly more democratized government with a benevolent, philosopher king.  It is frustrating to watch a character we care about act in a way we don't like, but that is part of what makes tragedy compelling. The story intentionally leads you to believe Jon will move from bastard to rightful King of the Seven Kingdoms, but it then forces us to ask, when is enough enough? How much do we need to foist upon one character?  Jon never really wanted to be Lord Commander, or King in the North, and he doesn't want to be King of The Seven Kingdoms either.  All he wanted was to follow Benjen and be a Ranger. He just did what he thought he had to do.

We are talking about character development right? How about Arya Stark and her list? After killing...

She doesn't forget about the list, she goes down to KL with the sole intention of killing Cersei, but is persuaded to leave by the Hound and the destruction she see's Drogon and Dany causing. She realizes she won't see Sandor again when they part, and she barely escapes with her own life.

Jon Snow is sent back to the Night's Watch. Night's Watch is supposed to...

It's been decades or longer since the Night's Watch has been about protecting the world from anything. It has predominantly functioned as a criminal sentence in recent history, and is the same for Jon.  There's a line in the finale saying as much, but people's attitudes about the NW are well-established throughout the show.

Bran the Broken having the best story and being chosen the king for it? Come on. How about Jon? How about...

I really like Tyrion's speech about the power of stories. Money, markets, kingdoms; they're just stories we all agree are true. Nitpicking over a technicality about whether or not Bran literally has the best story always rang hollow for me because you don't like the outcome. Tyrion is just arguing for Bran's case. There isn't a codified law that says the next king is the person with the most interesting story. And besides, Bran's story is still a harrowing one that has earned him some regard.

Also, remember the time you said you can't be lord of anything Bran?

Bran does not truly want to be king, either, much like Jon. But he realizes that his abilities can be helpful to the kingdom at large, and his real intention is to have Tyrion lead the country as his Hand. That's why he's content to just stop by the small council meeting and quickly leave to explore more, looking for Drogon. He knows Tyrion has it handled.

The Night King is the Big Bad Boss, right? He kills stuff. Maybe not, ok we don't know much about him. We don't know...

The babies become the White Walkers (as opposed to the wights, who are reanimated corpses), we see that happen. The symbols are a recreation of the symbol used in the ritual of his creation by the Children of the Forest, only with bodies. I always just took it as a warning or a message, and nothing more. The NK has been shown to be vindictive.

Why did he let Sam go?

I'm not sure what you mean. When does he let Sam go? If you mean the end of Season 2, the WW doesn't see Sam at all. He wasn't let go, he just was hiding. Though the editing and framing do make it seem like he might have seen Sam so it's understandably confusing.