I've been using the new YouTrack 4.0 agile boards for a couple days, and I've generated a short list of features / annoyances I find with the system. Generally I really like it, but there are a few things that are a bit wonky for me, and the best way to get these issues raised (and possibly added to their backlog) is to post!
On the iteration dropdown, searching for a sprint that doesn't exist should ask if you want to create that iteration
The calendar should show two months instead of just one so that you can plan iterations across month boundaries without having to swap between months
Choosing a start date farther in the future from the current end date should automatically move the end date to the day after the start date
Adding an iteration should assume that it will start the first work day after the previous one ended (work week configurable)
New task creation should have a save keyboard shortcut (or if there is one, it should be more obvious)
When I switch between views (minimal, compact, detailed), I don't want the screen to skip around if I have a lower story selected
Hard to find the backlog, hard to add stories to the backlog, hard to identify what qualifies a task to be in the backlog (from the interface).
Hot key for new task and save task confusing. One uses ctrl, one uses alt. It's like finger twister.
Difficult to / cannot select multiple (ctrl+click) stories and move in groups.
Ok, time to get off the soapbox.
Thanks for the awesome release guys!