I think it ultimately boils down to the game being niche with a cool hook and people aren't willing to accept that. They want the cool hook but also have the game tailored to them.
The hook is definitely fantastic. I mean, who doesn't like outer space? It's a fascinating subject. I think a lot of people got carried away by the concept, but it's a niche game made by a tiny developer. I'm actually surprised at how much they've actually put into the game, not disappointed that a feature set that would rival a game made by 200 people aren't included.
That first reveal I wasn't sure what to expect. I actually wasn't familiar with Hello Games (I've still never played Joe Danger, even though I own them from PS+ I think). The reveal had me at hello, though (oh god, no pun intended, I swear!). Science Fiction? Space ships? Aliens? Being able to travel from planet to space and back again? Yes, please. I then ate up every little bit of No Man's Sky information I could find, and I went through my period of disappointment with the game once I learned that there wouldn't be a lot of features like quests, NPCs (prior to them actually revealing NPCs years later, which was a pleasant surprise), no co-op, etc, etc. I just tailored my expectations to the size of the team, and what they've shown so far. Surprisingly enough, it didn't diminish my excitement for the game. It's still exactly the game I dreamed of, and then some. The biggest roadblock NMS had for me was its procedural nature. I hadn't seen a game attempt this level of procedural generation before, so I was skeptical. After watching all of the various tech videos and interviews with Sean where he or Innes McKendrick demo the proc gen tech, I was sold, and that was the last of my worries. A lot of people think it's just going to be a dog reskinned yellow, or blue, or red, or purple, but still the same dog. I think people are going to be blown away by the procedural generation in this game. It's got me the most excited, honestly.
Even now, people are still speculating on features and content that more than likely aren't in the game, and to me, that's just a recipe for disappointment. There are a lot of ways a game can disappoint you. I just try not to let my excited expectations be one of them. I'm on board with the game from what I've seen and heard in interviews and trailers. Outside of some awful jank (like piss poor controls or something), I can't see myself being let down by this game.