What Makes a Great Aircraft Add-On in Flight Sims?

When a new aircraft add-on is released for a flight simulator, some turn out amazing while others can be a bit disappointing. For some people it’s all about system depth and realism, while others care more about performance, visuals, or how fun the aircraft is to fly.

In your opinion, what really makes a great aircraft add-on? Is it realism, good performance, detailed systems, or something else?