Hi!
sendfile() and Apache didn't. Since 2.0.44, Apache has sendfile support, and our storage servers use negligible CPU anyway, they're disk bound. So it's pretty likely that we could put apache on them with no significant performance loss.
Well, there's another side issue - the memory used. Event-model based server doesn't have to spawn that many children, and even with all the copy-on-write efficiency, memory footprint of 2.0 apaches would be way higher. Of course, 2.2 has the event based model too. And memory wasted = memory not used for filesystem caches.
But yeah, at the time lighty was the champ, and now it probably doesn't make much difference.
I also liked lighttpd configuration simplicity :)