As for 1.0 (forgive my playing the devil's advocate here), why stop at 1.4? Assuming Java 1.5 will be released by the time FOP 1.0 will be released, why not base it on the latest and greatest? Would that offer any benefits? What problems might that lead to?
Well, 1.4 is out since nearly two years now (counting the usable betas), and will be stable for nearly two years or more when 1.0 is released. This means it will be available for even pretty obscure environments. The only users stuck with 1.3 and earlier will the ones where upgrading to 1.4 implies severe side effects, mainly upgrading essential software services, the whole OS, or even hardware upgrades. From what I heard, development efforts are meanwhile firmly based on 1.4, everything based on 1.3 is strictly maintenance, with a gradual migration to 1.4.