Not to criticize those who have worked on project management systems in the past, but I've found that most systems are generally pretty painful for me. Generally the project management "systems" that work best for me is something like a shared Evernote checklist or org-mode, but those aren't nearly as good for clients/teams to use.

One of the things that I've often encountered is a granularity/association problem: tasks often become either Omnibus tasks where there's a shitload of stuff jammed into a single task, or a spread out mishmash of related tasks that don't have very good linking between them.

A good example would be a web development feature. I'm going to do the backend, someone else is going to do the frontend. Those are two pretty distinct tasks, but there's a lot of shared communication there. And most PM systems I've used don't have a (good) way to link those together. Mostly in my experience, 3rd party tools end up getting used, with links to shared wiki/moqups/google docs/dropbox whatever.

If I need to have 7 tabs open (email, slack, trello, moqups, google docs...) to get all the information I need to figure out what I need to work on next, my PM system isn't serving me very well.