Its just how unspectacular (in a very positive sense!) it is.
Yes, i know, there are tons of really lightweight and / or userfriendly Linux distros out there, but most of them do have the same problem most modern operating systems have: They just "feel" heavy.
The reason is - in my opinion - that all modern OS inherited the genes of big, multi user server operating systems... Haiku is different: It is a classic single user system intended for desktop use. Even without nostalgia for classic systems like Windows 95, TOS or classic MacOS, Haiku just "feels" lightweight and don't stand in the way of the user.
I was also really surprised to find a small, but nice software collection in the package-manager... including an (on first glance) really usable office suite.
Hmmm, that does sound really intriguing. Single user experience, the way desktop computing is supposed to be. Looks like I have a weekend project to see if I can get it running in VirtualBox and try it out, thanks for the recommendation!