My current best guess for the deciding factor (for me, at least) of whether something feels fun is the ability to temporarily forget about long-time goals while doing it and focus only on the task at hand.

The related common wisdom goes that procrastination can be driven, among other things, by fear of success or the failure to break down the task into subtasks. I find that it doesn't have to be about fear -- I am equally capable of being distracted by excessive long-term optimism. And simply coming up verbally with a list of subtask isn't enough. When I keep regularly zooming out and glancing at the bigger picture, the small subtasks don't register with my "motivational sense". I can congratulate myself for accomplishing a subtask from the list but it won't be accompanied by a gut feeling of achievement and won't provide reinforcement.