Pros
Atlassian still knows how to have fun and is a workplace that doesn't take itself too seriously (despite producing awesome things). Forget the dry meetings and layers of red tape, fun is built into the Atlassian values and it's employees don't forget that. Management at Atlassian are very hands on and there is really no sense of hierarchy (today I walked by our events room as our General Manager put out all the chairs alongside his EA). I've worked at 'Best Places to Work' before but Atlassian has nailed it's culture.
Kontras
Be ready to be open! Everything is open & shared at Atlassian by default and you're encouraged to give and accept feedback from all parts of this business on your ideas and your work. It takes a bit of getting used to, but it works :)