I'm too young to remember BeOS but I've taken a superficial look at Haiku and I don't get the hype. What made BeOS so special? How is it different from GNU/Linux or BSDs?
I liked it because it was very fast (I would always demo the startup time vs. Windows) and had a clean, macOS-inspired UI.
Super responsive—running ten things at once, on a Pentium 90 or PPC. The filesystem metadata was neat as well, and though we have these things today, it was unique in the 90s.
Keep in mind that BeOS was released in 1995.
BeOS had pervasive multithreading and a slick UI. The BeBox had dual CPUs, a novelty at the time and many years before multi-core CPUs.
Linux was still very new, and didn't have much of a GUI at all (maybe basic X, but this was long before Gnome, KDE, Englightment, etc)
Mac System 7 didn't have protected memory or preemptive multitasking.
Windows 95 was brand new and while a big improvement over Windows 3.1, was still very prone to crashing.