> What exactly does Eclipse provide for IDE developers?

Eclipse provides a gigantic framework for application and plug-in developers.
A lot of features come for free, like:
- Text editors, which support syntax-coloring (with content
partitioning), completion, hyperlinks, code folding, hooking up with
an outline, ...
- Local history, which allow you to restore a previous version of a file
- Content versioning system
- Views, which can be moved, minimized, closed, restored, ...
- A building framework
- A debugging framework
- Progress monitors (which show a progress indicator and allow
interrupting the task)
- Perspectives (a way of organizing views by tasks)
- Preferences, which are automatically saved and restored
- Property pages
- Wizards, for creating new projects, etc.
The list goes on and on...

> And what is rewriting camlp4 in Java like?

There is no need to, camlp4 (like any other command-line tool) can be
called from Eclipse, and its output used in OcaIDE.