In short-You are pretty much going to be in the boat with the rest of the laptop users around the world of "I am going to need to play this on a desktop" which is really what games are still meant to be played on. Now, unless you have a really new Alienware $2000+ computer, you are grouped into this category.
desktops? doesn't Blizz read the trade mags? desktops are like 8tracks! Acouple more years and hardware manufacturers won't even be making desktops....
Are you serious? I mean, if you were exaggerating some, please let me know. I have been in the IT world in the military and civilian world since I was 12 years old (obviously, Military started at 18 for me) and now I am going on 27. Desktops being like 8-tracks? Shifting the desktop out of mainstream IT and gaming is going to be like trying to get the American Economy to get rid of the dollar currency. A desktop, hands down, will get you the most power for your dollar. I understand laptops allow for mobility, but let's be honest. If you are going to play Diablo 3, you aren't going to go to the local starbucks and sit and play for an hour and hop off (at least not most gamers).
To get the most bang for your buck, you will be wanting to get a desktop. Laptops don't have the high powered capability that desktops have (nor the heat dissipation). There are numerous articles, research documents, and educational resources that point to desktops outclassing laptops.
Anyways, as for your statement of comparing Diablo 3 to WoW. That's comparing apples to oranges. Yes, they are both games. However, they run on different engines and the specs will be different than WoW. Otherwise, they could just have a universal spec for all their games. Unfortunately, that is not the case.