On the last day of the dev summit the ESRI folks do a closing session over lunch where they recap the week. I think my favorite stat was that we drank about 4x more coffee than we did water throughout the week. I know I was slamming afternoon coffees to get me through the late afternoon sessions!
- The 4x API is much improved but there is no quick or "easy" migration path for apps written with the 3x API. You need to read the compatibility matrix that ESRI provides and plan accordingly.
- The 4x API is built around specific programming patterns like the Acessor class and watchable properties. Learn these or you will be "Fighting" against the API. The online docs are great for getting up to speed.
- The 4x API is very extensible but you need to know where the hooks are. If you are integrating with other frameworks you need to understand the approaches for working with the Dojo loader and Dojo build system. Again the docs are your friend here.