The fact that you wouldn't distinguish between a regular PC and a professional workstation perhaps points to what I might consider your apparent ignorance regarding hardware of this class, be it Apple or otherwise. The Dell Precision 690 is for example rated for continuous operation in a maximum ambient temperature of 40 degrees centigrade. Most "PC's" would give up before that and would not run stably at that temperature. What you're getting with a workstation over a desktop are various factors: Increased expandability, more reliable operation and space for more storage. Expandability and enhanced serviceability is a de facto requirement, as well as more robust build and better support. It is actually built differently to a 'normal' desktop, even a higher-end one.
The thing is, you probably don't know better (and neither do some journalists who look at regular desktops all the time) because you haven't come across this class of hardware in the PC world. Apple's genius (as such) is marketing and in this case the recent practice of upselling users to a true professional workstation instead of a regular upper-model PC. The Mac world hasn't had a 'normal' desktop since the Amelio days - even the G4 was a quasi-workstation for the time. This kind of gear has to date usually been used by engineering professionals, and isn't something that's normally considered for the mass market on the PC side at least.
There has been a slight change recently with the advent of very high performance gaming PC's - which have also taken on workstation characteristics. The Dell XPS 710 is one such example, equipped with a 1KW supply (as the Mac Pro), 4 snap-in HDD drive bays in addition to front panel slots and enhanced airflow management around all componentry.
By that token, it was laughable for Mac-bashers to compare a G5 to a (although of course the Intel Switch has exposed the megahertz myth given the huge jump in Apple performance with the new processors as opposed to the relatively incremental increase in Windows machines) higher-end desktop for that reason - you were comparing a workstation against a desktop and the comparisons weren't equal. It is however equally laughable for someone to berate a long-time user of professional workstations for critically examining the Mac Pro from that standpoint.
The aesthetic work is great, I am not taking that away from Apple. However things such as the drive bay design is inferior even to the simple snap-in plastic carriers from Dell from a functional point of view. And as a professional workstation, function should be utmost in the minds of its designers. By being sidetracked by design, Apple have created a 'wow' machine which definitely takes the first impressions crown from the aforesaid 690 - but clearly doesn't work quite as well in a production environment.
The practice of bundling a low-end consumer service contract with a professional workstation these days is also lackadaisical. Anything else, or is that all?