Okay, for pricing, $300 is too much, but as others have said, Nintendo is just keeping it's price from the competition - under $300 is pretty much a given since that's what the base 360 costs. If you want to go crazy conspiracy about it, there are still elements of the system to be revealed, perhaps they add $100 or so to the cost. I doubt it, though.
On the topic of "niche" markets: people love to point out that adults are now playing games, but how many of those adults were already videogame players as children? I'm guessing at least 80%. The market is basically only expanding because new gamers are being born - and it has lost some older gamers (my dad). I think the Revolution has a good chance of selling to people like my sister, or maybe my dad: former gamers or ultra-casual gamers who would never spend $400 to own a complicated "mainstream" system like Xbox 360 or PS3.
That's also why I think Nintendo will ultimately launch around $200 US: it knows that the system has to be cheap to sell to the non-gamers it wants to capture.