Yes, you will need to have an account to save your game. But I guess all web games require that. RPG Playground will not require you to make games, so you can just play games if you want to.
People can freely play without registering, but when saving, it asks for email and password. You can compare it to Kongregate or Steam or anything else, where you both have players and game developers. Players still need a login too.
Below is the working version of the new welcome screen. As you can see, you can create, but a huge focus is now on playing. You will get this screen without registering, and you can play without registering.
A direct link to your game will still start it right away, without registering. So it should be fine.
Remark that I want to bring RPG Playground to Kongregate.com this year. So that means a lot of players will be able to play your games, without the need for a RPG Playground specific login (because they use the one from Kongregate). Saving etc. will also be possible there.