http://www.theverge.com/2013/2/6/3958162/valve-steam-box-cake
1. Gabe himself has already said he wants 3 tiers of Steam Box hardware, "good, better and best." Also remember it is simply a standard. How companies implement the standard will greatly affect the cost.
2. Not any more.
http://www.themortonreport.com/home-away/gadgets/gadget-review-amimons-whdi-stick/
That article is from 2011, and concerning wireless HDMI it states "Video quality was only negligibly lower than that achieved via a wired connection and showed no perceptible lag, stuttering or pixilation."
So if it was working in 2011, I imagine it's come up another notch or two in the last couple of years. Remains to be seen if it can be competitive with twitch shooters, but I don't think the problem's as insurmountable as you make it out.
3. Given that you can already launch most outside games through steam, would make sense to extend similar support to the linux version.
4. Perhaps, but this isn't for those people. Plenty of people out there who would want to play their PC like a console but haven't invested for whatever reason.
5. At this point an exclusive would backfire, and I'm pretty sure that's not on the agenda (unless Gabe has gone completely nuts).
Above all, assuming this takes off it could bring PC gaming to the console gaming masses. That would be awesome.