Input devices: mouse, keyboard, touch and gamepad support
Entity-component system: high level game object management
According to David Evans in his announcement on the PlayCanvas blog that the platform is now open sourced under the MIT licence
PlayCanvas exposes a game framework that implements an entity-component system, allowing you to build the objects in your games as if they were made of Lego-like blocks of functionality.
If you are thinking "OK, another game engine", it is worth pointing out that you also get access to a set of in-browser design tools that make use of PlayCanvas and allow you to work interactively.
The PlayCanvas Object Editor - what two chairs and a concrete barrier are doing in a game is a good question.
As well as providing tools to help you develop games, PlayCanvas also hosts them for you for free - but only if your project is public. If you want to have a private developement area then you have to pay $15 per month for a Pro account.
The PlayCanvas site has some games available for your to play and explore including Swoop which runs in both mobile and desktop browsers as is on billed as:
a great demonstration of what you can achieve with HTML5 and WebGL