One, Spotlight is just a search tool. They run a daemon in the background that monitors reads to the FS and updates an index [1]. In Haiku, indexing and querying is a feature built into the filesystem.
Secondly, Spotlight does full-text search and indexing whereas Haiku only indexes the attributes you attach to files.
Metadata plays a very important role in the Haiku world. You cannot fathom the extent of their power unless you use the OS. This is not a "feature" that was tacked onto the OS as an afterthought, the entire OS was built around this feature.
[1] I'm actually working on a tool like Spotlight for Haiku (http://code.google.com/p/haiku-beacon/), though development has been stalled since my PC went kaput.
whoops, forgot about spotlight. i stopped working on my newsreader before it became viable.
i've examined the spotlight apis from time to time, and it does look like it solves a lot of the same problems. having never really used it, i can't say if it does as good of a job as bfs indexes do or not.
i can say that the beos file system is much more amenable to database-like usage than the mac file system. bfs has indexes implemented as a first-class feature, just as important as filenames or metadata. indexes are managed by the file system itself. spotlight was kind of bolted on top of hfs+. its indexes are in user-level files.
how close does Spotlight get you?