I believe a wizard was mentioned at one point but I’ve no idea on the current status of that.
No more support for 1.x bindings.
Changes in name spaces which shouldn’t impact users but might depending on how deep into the weeds users have gone (e.g. getting Items from the Item Registry in rules).
now and the first argument to createTimer no longer use Joda DateTime and instead use Java’s built in ZonedDateTime.
No more PaperUI or HABmin, both replaced by MainUI.
A much greater emphasis on building a semantic model with your Items so UIs like the following can automatically be created.
A list of breaking changes will probably be included, as always, in the announcements. I think there will be a migration tutorial. And there is a good start to getting started docs at Getting Started with OH3: rewriting the tutorial - 1. Introduction