Hard to put into words without gushing.
Firstly it's huge. You can be a bounty hunter, pirate, explorer, miner, trader or get involved in macro/micro politics. There's literally hundreds of thousands of galaxies. Aside from Frame Shift Drive which allows FTL travel, everything is pretty realistic. Ships and space stations are built to a realistic scale. Some things are massive, some tiny.
Next it's difficult but in a rewarding way. It never holds your hand, the basics are easy but no one ever tells you things like optimizing your ships power modules to overclock the number of systems you run. Using and disabling flight assist, using lateral and vertical thrusters to complete crazy manures. Small commonsense things like instead of downgrading the size of your fuel tank to save mass, just running with it half full.
While there's a natural progression of upgrading ships, there's no "best ship" you can buy. All ships have neat roles they can play. A maxed out cheap ship (with a good pilot) can often trounce a poorly upgraded expensive ship . The upgrades you buy are 100%refundable and ships are ~90% (less because you can strip and downgrade parts), so there's no barrier (at least early on) to experimenting and trying out new ships.
Finally, if you pour some more money into it, I understand it's an experience almost unparalleled. I don't have an occulus rift developer kit but apparently it's amazing, turning your head to look out the windows in a dog fight. I've bought a Hands-On-Throttle-And-Stick or HOTAS (which hasn't arrived yet) to really capture the flight controls. Finally if you install voice attack, you can program complex macros to a single phrase like "punch it" to close all hardpoints and charge the warp drive to escape. Voice attack has numerous voice to speech options so you can have an computer that repeats the orders back, switch on your music player and you can even program it to banter for certain phrases. My dreams of building Gay Deceiver out of Heinlein's Number of the Beast is a real possibility.
Regardless of all this. I'm only playing it on laptop with a 360 controller and I'm having a blast. I can see it not being for everyone, but I have never been a simulator fan and I'm obsessed.
TL

R
-Lots of realism
-Difficult but rewarding
-Lots of different ships to easily experiment with
-Could be the closest thing to true Sci-Fi Virtual reality in years.