Bug 96917 [implementation] If two editors use the 2.x-compatible API, one or both doesn't work
At least one problem is that 3.0 installs partitioners for every editor that might possibly edit a file, but each editor expects its own partitioner to be installed. Leading to the famous white screen with blue line.
org.eclipse.jdt.ui/ui/org/eclipse/jdt/internal/ui/javaeditor/CompilationUnitDocumentProvider.java
private void initializeImages() {
				
				if (!fQuickFixImagesInitialized) {
					if (isProblem() && indicateQuixFixableProblems() && JavaCorrectionProcessor.hasCorrections(this)) { 
						if (!fgQuickFixImagesInitialized) {
							fgQuickFixImage= JavaPluginImages.get(JavaPluginImages.IMG_OBJS_FIXABLE_PROBLEM);
							fgQuickFixErrorImage= JavaPluginImages.get(JavaPluginImages.IMG_OBJS_FIXABLE_ERROR);
							fgQuickFixImagesInitialized= true;
						}
						if (JavaMarkerAnnotation.ERROR_ANNOTATION_TYPE.equals(getType()))
							fImage= fgQuickFixErrorImage;
						else
							fImage= fgQuickFixImage;
					}
					fQuickFixImagesInitialized= true;
				}
			}
			private boolean indicateQuixFixableProblems() {
				return PreferenceConstants.getPreferenceStore().getBoolean(PreferenceConstants.EDITOR_CORRECTION_INDICATION);
			}
			/*
			 * @see Annotation#paint
			 */
			public void paint(GC gc, Canvas canvas, Rectangle r) {
				initializeImages();
				if (fImage != null)
					ImageUtilities.drawImage(fImage, gc, canvas, r, SWT.CENTER, SWT.TOP);
			}
			/*
			 * @see IJavaAnnotation#getImage(Display)