(( You can skip to the TLDR conclusion at the bottom if you're just in for a quick summary ))
I've played many of the space 4X games out there including Master of Orion 2, Imperium Galactica 1-2, Pax Imperia 2, Galciv 1-2, SotS 1, Endless Space, Space Empires 4-5, Star Ruler, Lost Empires, Armada 2526, etc. and to me what sets Distant Worlds apart from all of these (and makes it a more than worthy addition to any space 4X fan's collection) is the plethora of autonomous elements populating the gameworld, combined with an intricate economic and logistic simulation that runs in real-time, making DW's galaxies feel "alive" and real to a degree that I just haven't seen in any other space 4X game to date.
For once the people that you're taxing for income are finally actively present in the galaxy - they're running a private economy, buying freighters from your shipyards, hauling resources to wherever they're in demand, carrying tourists to your resort bases, migrating in passenger ships from colony to colony to avoid high taxes, etc and they do that completely without your input - in fact you can't even control them directly but merely influence them (e.g. by designing better freighters for them to buy) which gives DW a bit of a Majesty-like charm. All of this isn't just a small sideshow either - these civilians actually make up most of the interstellar traffic in the game.
The part of your empire that you directly control is also a lot smarter than in other games. It's not a completely helpless child that needs babysitting with even the most basic tasks such as resource flow or self-defense.
Resource flow is handled by the aforementioned private economy *if* they have enough money to buy and maintain ships/bases after paying your taxes.
As for self-defense, even on full manual control, your ships (can) have individual AI that assigns them to civilian escort, patrol, defense, etc duties across your territory according to your empire's needs (and yes they're smart enough to refuel/repair without you). You just build them and they'll handle the rest *if* that's what you want or you can take manual control and have them follow your orders only.
In either case, you can leave them completely unattended because it's possible to configure their ship design to e.g. kite enemies with long range weapons, close the distance for short range attacks, board and capture hightech enemies for disassembly, etc. so that they fight exactly how you want them to even when you aren't looking. And it's always a glorious moment when your automated defense fleet first defeats a pirate raid without any input from you.
In fact part of DW's appeal is that when building your empire you're basically creating and nurturing a complex self-sufficient system (and as city builder games have shown us, that can be a lot of fun) with the aim of making it more efficient than the rival empires.
An interesting quirk is that you can automate just about anything from research to foreign policy, military operations, espionage, etc. On full automation, the game essentially plays itself. That'd be like watching a pure AI vs AI match in other games. Only here you can influence your AI by taking control of certain decisions. One possible playstyle is to only control one aspect (or just one fleet, or ship) of your empire while leaving the rest to the AI which makes it feel like you're just a minister/general/fleet admiral/ship captain instead of a supreme ruler.
The ability to automate things also makes the late-game mop-up period a lot less tedious. In other games, I often just quit once I reach a point where nothing can stop me because the game is practically won and the only challenge left would be to endure the tedium of rinse-repeat steamrolling whatever is left out there. In DW, you can finish up by declaring your wars, putting your military on full auto and watching your good hard work come to fruition as carnage ensues without any of the usual tedium of having to manually control all of that.
Naturally you also have most of the standard/good bits from other 4X classics too such as custom ship design, diplomacy, research, espionage, pirate factions, assignable characters (scientist, governor, etc.) with developing abilities, random events, hidden tech, abandoned ships around the galaxy and so on.
There are also some extras like the logistics system - in DW your ships generate their energy from fuel that'll limit how long they can travel or fight before needing stop by a gas giant's mining station or a spaceport to refill their tanks. I also really like the pre-warp starting condition where you're confined to your first solar system until you develop some type of warp drive due to the simple fact that your impulse engines are too slow and would take forever to reach even the nearest star.
My primary complaint would be that the research aspect which is exactly like the research in the Civilization series (and most other 4X games) is too static in my opinion. Since technological progress essentially defines the structure of how these games play out, if the tech tree is always the same, then each game will also start feeling the samey after a few playthroughs. Fortunately in DW, 1 playthrough can be very long - I'm 137 hours in and only played 2 games so far (though I play on full manual and like to pause/ponder a lot).
In this regard, Sword of the Stars tried something very commendable by giving the option of a randomized tech tree where the list of researchable techs differs from game to game so you can't always just beeline for e.g. full laser weaponry if that's not available in the current game but will need to resort to some other weapons that will also neccessitate different tactics, different ship design, etc. I'm not saying this was the perfect approach, but it was certainly more interesting than a static, never changing tech tree that only acts as a drag on the replayability of otherwise excellent games.
Of course, this isn't a negative point in the sense that most other 4X games handle research the same simple way (remarkably, one of the few exceptions was good ole MoO2). But I hope game designers will someday inject some dynamism and randomness into this most overlooked and outdated aspect of the 4X concept.TLDR:
All in all, the reason people keep saying that other space 4X games feel empty after having tried DW is in my opinion the abundance of intelligent, autonomous elements populating the gameworld, combined with an intricate economic and logistic simulation that runs in real-time, making DW's galaxies feel "alive" and real to a degree that I just haven't seen in any other space 4X game to date.
Some of these elements, such as the private economy's ships and bases, you can't even control, just influence. The parts of your empire that you can control are also smart and independent enough not to crumble without your constant intervention. You act as a supreme ruler here whose attention is only called upon for important tasks and not to micromanage every single ship, trade lane and fuel depot.
However while DW frees you of the unnecessary type of micromanagement, it gives you a million other things to manage on a more meaningful level. The depth is actually quite overwhelming at first and you should probably leave automation on until you learn the ropes. Initially it will likely intimidate you and feel chaotic to the point where you'll be imploring at your screen "Goodness Lord, there are people playing this thing completely manually?!". But over time you'll grow to appreciate the complexity and all the choices you can make when taking the reins from the AI. At the end of the day just how much you want to control is up to you (you can micromanage nearly everything *if* you want to), but regardless of your choice, playing DW is a refreshingly immersive, truly vibrant experience that's definitely unique in the space 4X genre.