Properties of the Objects model (Gforth Manual)

It is straightforward to pass objects on the stack. Passing selectors on the stack is a little less convenient, but possible.
Objects are just data structures in memory, and are referenced by their address. You can create words for objects with normal defining words like constant. Likewise, there is no difference between instance variables that contain objects and those that contain other data.
Late binding is efficient and easy to use.
It avoids parsing, and thus avoids problems with state-smartness and reduced extensibility; for convenience there are a few parsing words, but they have non-parsing counterparts. There are also a few defining words that parse. This is hard to avoid, because all standard defining words parse (except :noname); however, such words are not as bad as many other parsing words, because they are not state-smart.
It does not try to incorporate everything. It does a few things and does them well (IMO). In particular, this model was not designed to support information hiding (although it has features that may help); you can use a separate package for achieving this.
It is layered; you don’t have to learn and use all features to use this model. Only a few features are necessary (see Basic Objects Usage, see The Objects base class, see Creating objects.), the others are optional and independent of each other.
An implementation in Standard Forth is available.