At the moment we're fighting to remove all the legacy Sun systems from our datacenters, and love the chance to remove these old machines.
They're rock solid, and do a great job. Our databases still run very very well on them, frequently more stabily than newer X86 kit they're being replaced with.
1) Power usage is insane. The datacenter team reported the larger boxes (ie, 12U type beasts like this) use the same power as whole racks of the standard IBM/HP type pizza boxes we can replace them with. Modern Xeons are multi-cored/multi-threaded enough to compete seriously with the older SPARCs, and do a good job of it, without needing their own power station too fuel and cool them.
2) Parts are getting harder to find, and vastly more expensive. As they age the cost of supporting them sky-rockets, and with parts being harder to find if something breaks there is downtime to fix it. That's not a good situation to be in. Indivual parts for these old machines (eg. spare HBA card, etc) are now becoming as expensive as a new replacement system.