There are no dedicated servers for Nintendo Switch Online as of yet so players will have to play using on P2P gaming, which relies on the internet connections of individual players to host matches online, making it a little unreliable.
Remasters of classic titles will also only be accessible to those with a subscription. You'll be able to use these games for seven days offline before your Switch requires you to "check in" online and make sure you're still a subscriber. However, we must also consider that if Nintendo adds a way to save data externally, it will eliminate the necessity of one of Switch Online's main features - Cloud Saves.
Cloud saves will prove a main attraction too, ensuring your save data is backed up in case you lose or break your console - though we'd advise against doing either of those things. But for what it's worth, games that need the Switch Online subscription will be marked when you see them in the Nintendo eShop or on Nintendo's website. Previously, the FAQ stated explicitly that cloud saves would not be preserved if your Switch Online account was closed. As we explained in our Switch Online breakdown, one month of the service costs $4, three months costs $8, and a year's subscription costs $20.
After the launch, you'll also be able to buy Nintendo's wireless NES controllers for playing the retro style games.
My Nintendo Gold Points is a rewards system that gives players points for every purchase. In preparation, Nintendo Switch eShop is scheduled for maintenance starting at 5pm PT and will be unavailable for up to 3 hours.