What are the really important things for 1st person?

What you mention above I heard…, I saw …, I smell…, etc. are filtering words that put distance between the reader and the MC, however there are not exclusive to the 1st POV, there are an issue with any kind of POVs.

For any POV you should remove the filtering, whenever possible, to get the reader closer to your MC.

For example:

"I sat at a table, waiting for Joe to arrive. I looked out of the window, and I spot him at the street corner. I watched as he approached, and noticed the with long, hurried strides he took, and I felt scared."

Now the sentences can be edited to remove the filtering words. For example.

"I sat at a table by the window, waiting for Joe to arrive. It's not long before he emerges from the street corner, approaching the diner with long, hurried strides. His haste sent a shiver down my spine."

In the first example, the filtering words tell the reader what is happening, but in the 2nd one, the absence of filtering words makes it as if you are the MC as it all unfolds. I hope this helps.

What's really important for a vivid first person narrator? There is not, I'm afraid, any universal answer to that. It all depends entirely on the individual character of the narrator and on the nature of the story they are narrating. What they notice about their environment, about the people and places they interact with, about their own responses and thoughts, all derive from who they are and what's important to them.

Somebody walks into a crowded room: describe the room they are entering. It's going to be completely different depending on whether you're seeing it through the narrative gaze of a sophomore art student, a newly divorced socialite, a professional hitman, a programmer who's just been coding in a cubicle for thirty hours straight. They will perceive things that matter to them, ignore things that are irrelevant, make assumptions and inferences based on previous experiences and current goals. You, as writer, know what they know. You have the direct path to their perceptions, and you feed it to us. You give us not a generic crowded room, but their crowded room. The room and the crowd that only your narrator can share with us, because they are themselves and not anybody else.

If you're not allowing characterization to permeate the first-person narrative, you're doing it wrong.

Also, as Elle says, don't filter. Especially in first person it adds distance you probably don't want.

I agree with benbenberi, first person is all about seeing the world through the characters eyes and their bias rather than a neutral narrator and the more defined the difference is the more engaging it will be. As for mentioning things boringly it's hard to know what advice to give without any examples, but the most common mistakes I see are people repeating themselves or not being straight to the point. The phrasing of sentences is important, E.G. "I could smell the alcohol on his breath" is a bit long-winded where as "His breath stank of alcohol" is straight to the point. Also if you are just putting "I smelled this, I heard that, I went here" I would focus on one with more detail rather than focusing a little on three.

I think you need to have a good sense of who your narrator is, what is important to them, and what their overall attitude is about their situation and the story they are telling. These things will influence the narrative voice and tone. Narrative voice is always important in fiction, imo, but it's especially important in first-person (also in deep limited third) narratives. It strongly influences whether or not readers connect with your pov character and what their sense of the person is. There are stories with absolutely despicable pov characters whom readers love because they tell their story in such an engaging voice.

Another thing to keep in mind is the narrator's distance from the events of the story. Are they relating it years later, with hindsight and greater knowledge of their overall situation than they had at the time? Or are they relating it in the moment that it happens (or a few seconds thereafter). Or are they recording the story in a diary after each major episode, but as it unfolds (with no foresight of how the story will end yet)? Also, if the narrator is removed in time from the story's events, do you want to create the sense that the person is writing their memoir, or telling the story to someone who is in the room with them, or just reminiscing? Keep in mind that a narrator who is relating things that happened long ago may not have the same attitude and voice as they did at the time the story took place.

Remembering that everything they describe will be biased, depending on their character. How a self-centred person views things as opposed to a deeply caring person etc.