Addon SDK v2; Better video support; Prompt on close & lots more.
All the documented APIs in the v1 SDK are still supported in the v2 SDK.
No the APIS won't be the same and all add-ons ever created will need to be rewritten from scratch (+ many of them will be impossible to port), which is MUCH more tedious than fixing 1 line of code every 5 years if something broke by accident