Psion
Adventurer
I notice that a lot of people have recommended Traveller, but the Traveller universe has problems, most of which are legacies of its '70s design.
- Traveller was designed in 1977, based on SF that was aging even then. Its 1950s and 1960s SF tropes are sadly dated, and it no longer looks futuristic. It has become retro-futuristic, like Space 1889.
I disagree. Traveller's look isn't that different from SF of the 90s and 2000's... Babylon 5, Space: Above and Beyond, Battlestar Galactica and (especially) Firefly are all right up Traveller's alley. Traveller in no longer considered "hard", but it certainly is representative of a sort of space SF that many people enjoy.
- The original designers half intended Traveller as a generic game, and a lot of the canon for their universe was laid down by deliberately-vague statements about a semi-generic 'Imperium'. The fundamentals of how the universe works were never designed and, in important ways have never been settled. Important facts about how the Imperium works are either unstated or contradictory.
Some of us find the unstated to be a boon more than a curse, as it gives us room to create stuff to fill in, but doesn't leave us the task of filling in things by hand.
That being said, there is a huge body of work for the Traveller imperium. I've tapped into GURPS Traveller supplements like Humaniti and Starports to fill in details, books that (thankfully, not being a GT fan) I can pretty much ignore if I don't want to use it. Best of both worlds.
- Most of the published detail of Traveller consists of hundreds of planets that were randomly generated with Traveller's lame late-70s random world generation system. Connections between a planet's size, atmosphere, population, tech level, and starport are not quite absent, but are far too weak, and many of teh results are absurd.
This complaint, while overstated, is more of less true. It really only makes a difference if you are a stickler about realism in your planetary data.
Unfortunately for me, I sort of am.

There is the somewhat bigger issue of habitable world in systems with white dwarfs, but that data is less "canonical" and you're usually safe to pencil in red dwarfs in the place of white dwarfs.

If you are not concerned about using canonical data but are concerned with realistic planetology, seek out and purchase a copy of GURPS TRAVELLER: FIRST SURVEY. It is world gen done right for Traveller.

- For "convenience", in Traveller space is two-dimensional.
This is true, and is actually one of the major things that has me pining for a game that is sort of the "next generation Traveller". A game that seems will never be.
In short, I could only recommend Traveller to a Traveller fan. If you're unfamiliar, and worse if you and your players are SF fans, you will find it laughable.
That, I vehemently disagree with. Albeit agreeing with many of your point, you are picking at points that just aren't that important to most SF fans. Traveller has some flaws, but has benefits in that it is highly playable game that most players can relate with. I have introduced many players to it over the years.
I bought and downloaded Thousand Suns a little while ago, which certainly sets out to be the sort of thing you want. But I haven't been able to form an opinion of it yet because (1) it is too long, and the type is too small, for me to be comfortable reading it on the screen, and (2) bizarre choice of layout and paper size (pages side-by-side on 6" by 9" pages, laid out like two-page spreads) makes it unsuitable to be printed out and bound in sensible format on any paper I can get.
I had high hopes for Thousand Suns being the next-gen Traveller game I was looking for. And it has many, many elements to be that game. However, I am finding I am thoroughly unsatisfied with the task system. It just doesn't seem to balance well against the chargen system. And for that matter, I dislike the strange decision to make ability scores static.
Perhaps I'll spin together a replacement task system that does what I think it needs to do.