Looking to powerful environments like Eclipse for inspiration makes a lot of sense
As an experienced programmer who dislikes Eclipse, I'd plead with folks to please try to clearly define the use case which is improved.
I think beginners really are beginners. I run workshops for children and adults who have never programmed, and the Arduino IDE is so simple that the are succesful in 5 minutes. I would hate to create a barrier higher than that. They don't expect "magic" like completion, that is something extra to explain, and further time to explain what to do when it gets it wrong (e.g. they select the wrong completion).
I also think pwillard made a very good point; folks who want changes to the IDE should be lobbying the developers of the Processing IDE (though no reason to stop gathering the requirements here).
Maybe folks who want an Eclipse-style IDE could look into folding in the Arduino toolchain into Eclipse?
GB