Cameron, thanks for your feedback.
I'm on a 2011 iMac, with a DIY Fusion Drive (I added a SSD inside). I always had slow startups. I thought it was the custom Fusion Drive's fault, but now I know it was not.
Now I can't boot at all, just after applying the most recent 10.9.2 security update. This is a software issue. The iMac was running Mountain Lion Server, then I upgraded to Mavericks but did not upgrade to Mavericks Server. However there were remains of the old 10.8 Server app.
As I said, yesterday I deleted all those remains (booting from an external drive). However I forgot the most obvious thing: open that **** /etc/rc.server file and comment out line 14.
This morning I booted the iMac again, in verbose mode. The message about /etc/rc.server was not there, at last, but fsck was launched and took a long time. I had to go to work, where I am now, and I don't know if fsck finished its work. I'll let you know...