Encoding. Math. Renedering. Complex database operations. Complex string operations. etc.
Anything that is CPU intensive cannot be a web app.
A Web App method:
public snot Insert_Finger_Into_Nose(finger digit)
// Pick nose here :(
A Real Program:
private int Some_Really_Hardcore_Math_Stuff(int A_Number)
// Do something useful :)
Well, that may be a bit harsh, but there is a LOT of ground to cover before CPU cycles become cheap enough to run an intensive app on the web.
What totally freaks me out is that Outlook can chew CPU like there's no tomorrow, but Gmail works... That's the odd man out though. Outlook is just ridiculous.
It will all boil down to cheap math though. That's what all applications are - math.