A lot of people here simply don't realize that the x86 manufacturers are getting close to ~3.5, and that within a year mainstream PCs will be that fast. Today most PC boxes are ~2.5... And given that longhorn will be out in what, like 2 years... We should be around ~5 as a conservative estimate, with most users buying new ~4 Ghz boxes. This is an OS meant to serve the mainstream market for a few years, lets say 5... By then we're talking ~9 GHz (random guess considering how far, but the point is that is WAY overshoots the "recommended" 4-6 spec that was quoted). The terabyte thing is just the HD space that high-end boxes will have standard by longhorn release, and by its maturity, think multiples of that. The installation will prolly take a DVD or two.
Remember how OSX could never have a chance in hell to run on a 1 yr old box, let alone being slooooow on new ones (bar optimizations brought by 10.x releases, we could compare this to longhorn betas in a year's time)? Thats right, same situation.
The truth is that these specs are realistic. Now as for longhorn being actually "good", as a programmer, let me tell you that the idea of n thousand people working on one project is insane -- it boggles the mind, sure theres endless manpower, but try organizing something like that, its insane. Everytime a new programmer touches something he didn't personally code, something is likely to go wrong, and in projects of that side, I imagine that code changes hands quite often.
... god! it will take a while for that and the first one to do that will be apple when the macs reach 120 TB HD BTO 
