First and foremost the ability to continue playing games like Splatoon 2 and Mario Kart on line. It was free up until now.
You also get a growing library of NES games which is actually the same emulator as used in the NES Mini, they just ported it over. Someone has already cracked it so that you can add your own roms.
I'm hoping that they'll do the same with SNES, N64, etc.
They also include special offers that are not available to anyone outside of the Switch Online community. Such things as wireless NES controller Joy Cons.
Also Cloud save so if your storage goes belly up you won't lose your valuable save data.
I've read a lot of bitching about having to pay but come on, it's less than 2 bucks a month. I heard one person say that they can't afford it. If you can't afford $1.66 a month how the hell did you afford a Switch and games for it?