Support for unsupported database versions has been dropped.
We haven't re-implemented support for some object types that no one really used in the tool before - for example, operator classes and families.
We haven't (yet) reimplemented some of the tools that didn't work so well in pgAdmin 3, such as the graphical query builder or database designer (which was always disabled entirely by default).
The Query Tool and Edit Grid have been merged into a single tool. Over coming releases we'll be improving the functionality further to allow in-grid updates to be made to results from arbitrary queries (where a query is determined to be updateable). For now though, updating is allowed when pgAdmin knows the data source is a single table with a primary key.
The user interface is more flexible than ever, allowing tabs to be docked and re-arranged in more ways than previously.
We've spent time redesigning some of the UI paradigms in pgAdmin 3. Gone are the list controls with Add/Remove buttons, replaced with what we call sub-node grid controls that will allow in-grid editing of key values, with more detail available when needed in expandable rows.
We also spend time thinking about how to make it faster to use pgAdmin, by minimising the need to switch between dialogues, using searchable combo boxes and more.
The UI is much more attractive, making use of control groupings and expandable regions to make things more readable.