1. I'm an OS junkie. I love installing, fiddling and learning new OSes. I installed BeOS and used it for about 2 years (no joke) back in the day. LOVED IT. I even had it replace my Slackware instance for about 6 months as my primary system.
2. I love seeing OSes still actively developed even if they aren't popular. I think it keeps driving ingenuity and pushes other people. But, I don't get why we have to have rewritten older systems...lets try to advance OSes.
3. The interface has to go. It was clunky back then and now it is downright horrendous compared to nearly any interface we have. Gnome might feel like 2004, but this feels like 1997.
4. The biggest problem with BeOS back in the day was lack of applications. I don't see this being fixed with Haiku unless they have translation layer.
EDIT - One thing I forgot to mention about BeOS back in the day. It booted in about 15 seconds. It was amazing. Right now the fastest I've seen is an Ubuntu box getting from start to internet in about 7 seconds w/ a SSD etc but for 1999/2000, it was amazing. Astonishing, really.
I would also add in this edit that I think the state of UI on current OSes is a bit disappointing. I think Ubuntu is trying w/ Unity and even that has a ways to go before I see any real innovation in interfaces. It feels like we've got a modality that worked 15 years ago and we've sort of massaged it, but haven't really innovated or pivoted on the concept too well.