Long story short - I've a very pleasant experience while trying Youtrack now. But let's take a closer look!
I kick ass withAJAX-based issue management system
The command-line with auto-complete. Sometimes I get really lazy and just unwilling to use the bugtracker as I have to use the mouse and click over several screens in the bugtracker's UI to get the job done. With Youtrack, I can make queries in the command-line that has autocompletion, which is awesome!
Project settings and administration. It took me just a couple of minutes to configure a project and the users. And also I figured out very quickly how to the user roles work. Considering I'm just an ordinary user for this type of software - this is a good sign!
Shortcuts are configured just like in IntelliJ IDEA, which I'm being a big fan of. I think this is an awesome feature for any IntelliJ user trying to use Youtrack.
The UI did not cause any animosity during use for me. Also, you can make screenshots just from the form where you report an issue - a very handy feature I have to say.