In E4 we are trying to address some of the main pain points RCP-Application developers faced - Their application looks like Eclipse and the theming/presentation API is not flexible enough to modify all aspects of the UI (e.g. Toolbar/MenuBar, ...)!
The first step to make your application look different was that we introduced the concept of declarative styleing (CSS).
Let's take the demo E4-Photo-Demo-application as example.
This is the application without CSS:
This is the application with CSS:
But like in other aspects E4 goes further and allows you to exchange complete widgets like the application-shell, the menu and toolbar, the Stack-Folders (the standard theme uses CTabFolder) resulting in an E4-Demo-Application like this:
You want to know more about how this works and how the internals of the workbench make such things possible? Then I'd suggest to you attend the EcliseCon-talk about the modeled workbench Boris and I are delivering at EclipseCon09
But E4 goes even further! It allows you to plugin every widget-technology you want to! The internals don't care whether your application gets renderered by QT, Swing, UFaceKit you name it. So I've done that and here's my UFaceKit-Demo-Application built on top of E4:
Makeing UFaceKit a possible renderer of E4 opens the doors to all technologies supported by UFaceKit (Swing, QT, GWT, ...) and its advanced DeclartiveStyling-Support