The main conceit of No Man's Sky is loneliness.
No Man's Sky comes out during the apex of 'Survival' games, such as Rust and DayZ, where the mechanics rotate around filling up different meters to survive and craft increasingly complex items as a means of thriving. The difference between No Man's Sky is that you aren't fighting off other players (you actually might not even be able to see other players), you're fighting against the oppressive size of the universe.
One of the main selling points for the game has been 18 quintillion (a billion-billion, yes I had to look it up too) wholly unique planets and the idea that if you spend a second on every single one, you would need to set aside about 5 billion years. The first thing that comes to mind when I hear that is that Borderlands 2 trailer where it claims there are "bazillions of guns." (This number turns into 17.75 million guns in the actual game... marketing numbers convert into real numbers weird)
The sheer difference in these numbers and the scale of the assets they represent should give you a full sense of what a technical accomplishment this is. In many ways, No Man's Sky mirrors the flash and maximalistic ideals of AAA development moreso than that of the current state of indie games. Where a similarly sized indie team put out a brilliant and ambitious game earlier this year in The Witness, that was grounded in intellectual ambition on a single island; Hello Games set out to create the ability to explore a universe.
But even though No Man's Sky promotes the idea of exploration and discovery, I feel what it invokes is less like Star Trek and more like the 'walking simulators' like Gone Home and Everybody's Gone to the Rapture. These games emphasize exploration in the wake of disaster, to try and piece together the remnants to form the story that preceded the game. No Man's Sky feels a lot like this same exercise. There are very few interactions within No Man's Sky, most of them are with aliens whose language you do no speak (though you can still easily trade and learn words from them) and seem indifferent to your existence. Most of your time is spent learning the lore of these species through artifacts and other indirect sources. You don't actively engage with anyone or anything outside of the crafting system and HUD; it's all player driven, not game driven.
That last part is important because it's a design choice that allows players to set and work at their own pace. You don't go to any particular planet, you go to one of the 18 quintillion planet you stumble upon. A notification doesn't pop up when you have enough elements to get an upgrade or refuel your ship. You don't get mission indicators and outside of the beginning tutorial you don't get a checklist to fill up. This is to really emphasize that it's up to the player to choose whatever they do next; this is a universe that is literally a creation of and for the player, it's not a stand-in for the developer's worldview.
Contrast this with Mass Effect 1: the planets you travel to are full of space stations and space ships, unique vistas, and a hodge podge of characters ready to give you juice bits of lore and gossip, quests that will drive what your Shepherd will do next, flavor text on all the weapons and items; everything comes together to remind you that you are a ship's commander in a space federation with uneasy allies from all types of species.
I want to disclaim here that I don't think that No Man's Sky would inherently be better if it were crafted in this way, I actually mean to show that this game is NOT looking to achieve the same things as Mass Effect. It's a different game entirely.
Sean Murray has said that he's afraid that players may only experience a small part of what the game has to offer, that the chances of encountering another player are nearly 0% (yeah, this has actually happened a lot already... ¯\_(ツ)_/¯ ), these all seemed like grandiose announcements of the content and playtime that lay ahead of players. In retrospect, after spending a fair amount of hours within the game, I realize that these were actually foreboding messages of the vast nothingness; warning me of the lonely path ahead.
The real disappointment isn't that No Man's Sky doesn't live up to my expectations, it's that it's so one-note in it's theme. Journey was a game that bushwhacked me with how well it creates a palette of emotions, including loneliness, which I'm not sure if I ever inherently felt from a game before it. But while Journey mechanizes itself to emulate loneliness and desolation, it also gives itself chances to take the titular journey with other players, breaking through the loneliness with chirping and flippant jumping, pressing through the simplistic puzzles with a buddy by your side, sliding down sand dunes and making vivid memories with people whose names I will never know (that's not totally true, one of their Playstation IDs was DEFINITELY TheAstroChimp). There were real moments of triumph in Journey, and they felt real because they were sparse and pointfully momentous.
The triumphs in No Man's Sky come when you are alone: mining, blowing up sentinels, and getting to new planets. Thanks to the 'Journey Milestone Achieved' notification they also come quickly and relentlessly. In fact I was already tired of them by the time I left the first planet. They reward you for doing any little task and it overall diminishes when you are actually achieving something interesting.
The most true analog I can think of when I think of No Man's Sky is the feeling I get from looking at the 'Blue Marble' photo. It's this beautiful, wonderful view that acts to reinforce just how small you are in the universe. It's immediately engaging and powerful; representative of the achievement that a small group of mankind can create when everything is rolling correctly, but it's also relentlessly bleak and incredibly overwhelming.
Which isn't to say that I don't like the game, I do. It's just that this sense of wonder and exploration drives players only moment-to-moment, there's an inevitable vacuum between your goals and there's very little about the universe that actively asks you to be a part of it. Even if this whole game exists only to be experienced, mined, and crafted by the player; an infinite amount exists outside of the player's reach, outside of the player's gaze. And when you're thinking about that on one of an infinite number of vast, emotionless planets, directionlessly mining another chunk of Zinc, that's the loneliest moment of them all.