Compared to what year? I know compared to my 15 STI, my 18 has bigger brake calipers, 19" wheels to fit the brakes, drilled brakes, an electronic center diff, a larger touch screen, thicker windows/more sound insulation, barely noticeable softer suspension dampening, more soft touch interior surfaces, a larger LED dash screen/boost gauge, slightly better looking (IMO) contrasting gauge cluster, a more neutral grabbing clutch pedal, adaptive LED headlights, LED boomerang running lights, a more aggressive looking front end (IMO), and a Recaro seat option.
If you're talking about changes from the previous generations, the rigid chassis and firm steering rack will be the most noticeable. If you're looking for any real changes to the drivetrain, you won't really find any. Ride quality and interior quality has improved from previous STI's, but still leaves a ton to be desired.
I have a really odd passion for the flawed EJ motor and it's beautiful rumble noises, so I'm pretty dedicated the to the STI. That said, I'd look elsewhere all day long. I paid $40,000 for mine OTD with the options I wanted. For that price you can get your hands on a Ford Focus RS, a Honda Civic Type R, and a Golf R. Honestly, all of those cars are much better reviewed the the STI and they're said to be much easier to live with. The modern STI is very behind the times. I stayed with this model, because I like the specific feel it delivers. If I cared about which car was the better performer, I wouldn't consider the STI. On a circuit track, which is what it's meant for, it can be slightly edged out by almost any competitor in its price range and it's very rigid. If you're looking to hammer the car down your local roads, which I don't suggest, the soccer mom cars like V6 Camry's/V6 Accords will make you feel silly when their kids borrow them and keep up with you. You'll feel even worse when the basic bitch Lexus and BMW's **** all over you.
That said... Good luck.