How would it work at a wi-fi hotspot? The Wii U GamePad doesn't process any information, it just recieves it. So unless the Wi-fi hotspot has a Wii U and you sync your controller to it, it won't work. Same way with streaming. The Wii U console handles all of the actual data, not the GamePad. So taking the GamePad to a wi-fi hotspot or anywhere without a Wii U system, it would be like taking the batteries out of your 3DS and expecting it to work.
As for the brightness, the videos at E3 showed the screen being very bright.