Premium features! Like 3D Vision, which leads to 120+ Hz gaming, which leads to G-Sync and ULMB. Or stuff like Shadowplay, all the HBAO and AA techniques, and of course a focus on smooth framepacing over outright FPS. Then there are the drivers which actually let you do stuff like downsampling or SGSSAA...
Overall, Nvidia has a bit of an advantage here. But I'm not sure if you are unaware of alternatives or purposely leave them out.
3D Vison = hd3d
G-Sync = Free Sync
Shadowplay = GVR
HBAO and AA techniques, are you referring to GW HBAO? TXAA is a nice option though.
You can downsample on AMD now: VSR
ULMB is nice as an alternative to G Sync, but it's one or the other sadly.
You can argue which feature is better, but generally both companies have a solution. If you are hardcore tied to a Nvidia feature and know it isn't as good on AMD, then that's one thing. But truly those are rare people. I would recommend Nvidia if someone was very dedicated to 3D gaming, personally. But look at out own PC recommendation thread, where almost no one ever mentions these features.
All thanks to Nvidia who actually try to come up with new stuff that improves PC gaming. And who then actually make sure this stuff is used by developers (where did stuff like True Audio go AMD?)
How exactly does Nvidia make PC gaming stronger? What makes PC gaming stronger is the developer focusing on making the game specifically for the PC, which hardly any of them do anymore. Taking a console port, adding GW features isn't proof that the developer is dedicated to PC gaming. RockStar reworking GTA V shows they actually did care. There are plenty of Nvidia GW titles which are 97% console ports through and through and no amount of TXAA or PhysX can hide it. Injecting code and forbidding devs from modifying it just hurts AMD, Intel, and Nvidia users (some people paid a lot of money for a 780 and want to keep it).
Are you going to say Nvidia buying Ageia Physx has helped the technology? By the very act of Nvidia acquiring Ageia, they have destroyed any chance of next generation advanced physics effects for all PC gamers. Since that acquisition, PhysX remains DOA except for Batman games and the BL franchise. Almost a decade has passed and so far we've hardly seen any progress with PhysX, thanks to its locked/proprietary nature that alienates Intel/AMD users. If Nvidia allowed AMD/Intel GPUs to run PhysX, PhysX would have exploded in games by now.
GW and PhysX are basically the exact same thing - it's a process of vendor lock to try to make your products look better than the competitors (and your own older products, Kepler) at the expense of everyone else in the market who doesn't own a new Nvidia product.