My biggest problem by orders of magnitude has been lack of guidance on process and habit forming. No matter what features the software package offers, I’d appreciate clearer answers to questions such as: when do I review the backlog of tasks? When and how do I prioritize? How do tasks and calendar entries correlate? Can they overlap? (Sometimes? Always?) Am I supposed to allot calendar time for every single to-do item? For some items? For none at all? (If not: how to ensure that I’m going to get things done?) What’s the lifecycle of the backlog? When do I decide whether and when to drop a task from the backlog? Are tags (or categories or folders) useful, or do they needlessly complicate things? How do I go about forming a habit for the review process?
From my experience, it’s all about the process, not the technology. The former is surprisingly missing from most to-do apps.