For me a blocker is everything which is just standard over the top science fiction. Forget black holes, faster than light travel, wormholes, alien civilizations etc. Let us explore stuff which makes sense in reality!
- Let me land on a moon like Io with vulcanic eruptions, toxic steam vents, earthquakes and strange gravitational effekts because it's to close to a large mass (like jupter).
- Let me land on a hot planet like venus, where I need special equipment to prevent the whole spaceship/probe from melting away.
- Let me land on a ice moon like europa and dig into an ocean under the surface. If I find some strange primitive life forms there, that would be awesome. But please please no humanoid alien civilizations or warp drives or interdimensional crap... We have other games/movies for that.