Itanium - the solution in search of a problem. I'm still waiting for those 64 bit apps. Notepad-64 gets a bit old after awhile.
Why? Because all those 64-bit pathways make the chip bigger, and bigger chips are slower chips.
Of course, some media instructions are good to have as wide as reasonable, and operating systems will use more and more RAM as disk caches. But in general, it's like renting a limo when a Ferrari would do.