I pretty much love YouTrack. It's a huge step up from JIRA.
There is only really one thing I dislike about it. The whole point of Kanban is we all should live on the board(s). So I want to be able to glance at the board and see what the hell is going on at all times. Furthermore, if I do drill down on something, I want to get a quick sense of what is going on with that ticket. YouTrack is very clumsy in this way. If you were to never nest anything, YT would be ideal, but of course we know that is completely ludicrous.
Worst of all though is that nesting leads to a whole hellfire of other issues. Deeply nested stories pretty quickly make the system unusable: actually, never mind the word deeply. Even something like this:
makes the system pretty useless. Say this epic was in the backlog and you just wanted to work on the task. What happens when you assign that and set it to a board and then set the status to In Progress? The whole furball gets dragged out and you can't really see why, unless you go on an archeological expedition.
Finally, the depiction of related items is really wanting. Every time I go into a ticket that has sub items, I have to open that stupid view? Really? And even if I do, I don't really get to see what is going on with it, again I have to investigate.
I hate to sound like Debbie Downer here but these issues are serious enough to be undo a lot of the positive work that has been done. The best thing about YT is that you really can stay on the board 80% of the time. But until these issues are resolved, the required departures will undo the convenience gained.