Responsiveness, namely. While OS X was growing up (it didn't become seriously usable until 10.3 or 10.4) I missed the lightning-fast UI of BeOS. Nothing slowed it down. You could go something, and let it chug away at it while you did something else. Everything felt light or responsive. Applications started instantly, it had Spotlight-quality instantaneous searches built into the filesystem (and none of this "indexing" crap spotlight likes to pull).
Also, it had the absolute best-written SMP support I have ever seen. In Windows and OS X, it's glued-on by comparison. Fast, elegant, usable, simple, and powerful. OS X is nice, but it takes a lot more hardware power for it to manage to be as snappy and responsive. And even then, it lags in odd places.
The Haiku team have made good if slow progress, and I hope they continue to do so.