The trello UI is also great. If you don't need the back-end features of something like Jira, trello's limited functionality greatly reduced the friction to tracking your activities.
You can talk to it from just about everywhere including the terminal, that's a big reason I use it, it acts as a clearing house for all my stuff, I even use a chrome plugin to store "bookmarks" to stuff I've clipped.
E.g. being able to make layout changes(have lists that span columns for example, or cards that show more of their information on the cover). Or more flexibility in colour changes/background, or ability to style cards.