From a technical perspective, I would argue that a DDD project is nothing but a clear and protected domain.
Working on lots of legacy code though, I observe common mistakes to identify what is inside the domain, and what is outside.
Your data model is not your domain model
It is a really common mistake to use a data model as a domain model. I think ORM and CRUD have their part of responsibility in this misconception. A data model is a representation of the model optimized for data storage.
The data model has the responsibility to store data, whereas the domain model has the responsibility to handle business logic. An application can be considered CRUD only if there is no business logic around the data model. Even in this (rare) case, your data model is not your domain model. It just means that, as no business logics is involved, we don’t need any abstraction to manage it, and thus we have no domain model.
Your view model is not your domain model
Another common mistake is to add just one layer between the data model and the view. This layer has different name (Presenter, Controller, ViewModel…Whatever) in different patterns and languages, but the idea is the same: abstracting the view using code.
The mistake is when we believe this is the place to add business logic. This layer has already the responsibility to abstract the view, we don’t want to add any other responsibility.
Your data model should not notify your view
I go crazy when I find a model object with the responsibility to notify the view when some data changed (often using databinding). It literally means that this data or domain object, has also the responsibility to manage the way data are displayed.
As usual, it is all about separation of concern.
Your domain model should depend of (almost) nothing
The domain model is a set of classes or functions, group in a domain layer, containing all the business logic. This layer is absolutely free from the infrastructure. It should be a pure abstraction of the business.
A domain model is still valid if we change the way we render the software. A domain model is still valid if we change the data storage. A domain model is still valid if we decide to stop using our favourite Framework.
A domain model depends of one thing and one thing only: the business.