Well, if it was for WoW - the hardware components would be fairly cheap - you'd just need to throw gobs and gobs of memory at it (less if you could reduce the O/S overhead. The GPU side of things would be relativley inexpensive - throw in a keyboard and mouse - and I think you could easily do this in a SFF case...hmmm.
Limitations
Need for lots of HDD space - a given, WoW's not particularly small...and each expansion and 1gb+ patch seems to get more and more re-goddamn-diculous; looking longer term...you'd probalby need a terrabyte of HDD space for Wow + 2 YEARS worth of expansions...then throw SC2 or D3 at it...I think that may be conservative
You'd loose people by the boat load if you made them use WoW voice chat - everyone uses Vent or the equivalent because the in-game voice option is just such a pile. I'd be really suprised if they'd let you install other software on their little box...though they could just buy Ventrillo i suppose
But that brings up add-ons...one of the bigger draws to playing WoW is it's user-interface customizability - I have serious doubts (and if I were Blizzard, reservations) about allowing third party addons on my little "solid-state platform" that was supposed to be unified...especially when those little addons get raging amounts of use (both in the public...and memory consumption)
Blizzard just tends to disagree...and flat out lag behind when it comes to certain add-ons - and the damn game would be unenjoyable without some of them. Look at Outfitter, for example....how long did it take Blizzard to implememnt a quick way of changing gear-sets ...4 years? For classes that constantly change gear (which is what...like 20-things) 10 times an a half hour....stuff like that would get real old quick...again, necessitating 3rd party add-ons
The other big thing that you'd like to see is the implementation of a higher end experiencing...but that comes with higher-end hardware requirements...and that could get absurdly expensive on the hardware side. Even at modest resolutions....like 16*10 - you're looking at a little rough and tumble box that can put out the game at the highest resolutions and settings - and that's without turning on just friggin' insane system hogs like high-quality shadows.... If we're talking about a console here, we're looking at people wanting to play this on HDTV's? So trying those settings at 19*10 again leads me to belive that the hardware would just be too darned expensive.
And you'd have to do a lot of work on the core interface to make it scale with TV resolutions - the text size thing alone...floating text bars (and TV refresh rates) is enough to make me want to cower in a corner.
Just food for thought!
Jack