The looks are the least important part. It works differently. Applications interact in a different way, there' APIs (especially around filesystem metadata) that applications and scripts can use. It's not necessarily that the capabilities don't exist on other systems, it's just that they're exposed and most importantly used in a different way. It's also a system that was built from the ground up to be GUI-based. The command line is there, but it's not the primary means of doing anything much.

There are BeOS/Haiku window decoration themes for at least XFCE and I think OpenBox. But looks are really not what Haiku is about.