Re: What about heat dissipation?
For a point of comparison, the average human brain is supposed to consume about 20W of power, about 20% of the body's 100W. So it's not that unreasonable an amount of heat for the Hololens to dissipate.
Unless your brain stops working while you're playing, it means doubling the amount of heat your head must dissipate. I suppose couch potatoes playing Hololens games will feel like they are getting a workout if their head gets sweaty, and ignore the fact that's the only part of their body sweating :)