I currently still use Omnifocus for part of my system because it is still pretty great, and has excellent cross-device support, but my gripes are:

- The context support is weak. It ideally would be replaced by a tagging system, but I doubt that's coming at this point.

- No support for making sequentially dependent tasks -- I can't mark task A to be doable as soon as task B completes. (I guess I could, if I do a lot of sub-projects with sequential dependency)

- In addition to this, subprojects are janky. They're just kinda awful to use, from the awkward indentation to the fact that the side-bar projects view is not actually a full tree, and just works with folders. This would need to be changed to also expand/be usable for sub-projects, not just for folders and top-level projects.

- The system maintenance tools are lacking. One of the core tenets of GTD is to make sure that every project always has a next step, and OmniFocus has no support for this whatsoever, aside from manual review. I realise that paper also doesn't do this, but it seems like something a digital solution should have.

- No sharing information, as you said, aside from exporting things as TaskPaper.

- Support for deferments, but not for Tickler files, which are one of the handiest things about a paper-based system.

- Reference and storage, per nature of OmniFocus, has to be completely separated, which adds jank. I realise this is massive scope-bloat, so it's probably not fair to complain about.

Things I really like and will probably keep even after I drop OmniFocus:

- The dedicated mail drop. I think even after I move away from OF, this is going to be something I keep.

- The "due" list view. OmniFocus pushes you towards using this a lot, even if it's not completely in the spirit.

- The good integration between my laptop and phone is going to be hard to live without again.

I hope this shed some light on my gripes and issues with OmniFocus. :)