In short? Everything.
Since this picture, I had made some changes:
I removed the far side of the case and mounted those hard drive stack 80mm's outside the case, which gave me a lot more room, ran the power cables between the drives like the sata cables are. When you get so much stuff in there, it becomes difficult to manage the cabling effectively.
4x160 GB SATAII in a RAID0 (~600GB), my OSes and games.
2x250 GB SATAII (@SATAI) in a RAID1, (work, writing, etc...)
2xDual Layer DVD RW (1 Lightscribe)
1xHigh Speed CD RW
(Floppy for the RAID drivers.
I also sealed up all the little gaps around the fans, above the motherboard and along the rear of the PC case. Air can only pass in through the fans, and can only come out through the fans or the Video card.