I spent 9 hours there last weekend.
Wifi isn't free Iirc it was something like 160 baht for 3hours.
Well there is free wifi, but you have to goto airport information and get login details and 15mins only, although you are meant to be able to log back in.
There's also plenty of Internet terminals which are meant to be free, but again I didn't use them, again 15min limit.
I sat in one of the restaurants for a few hours and had some proper food and a few beers, loads of shops to wonder around as well.
There is more than enough seating space around the place, but I don't remember seeing plug sockets, but then I didn't have my charger with me, so I wasn't looking.
There's also a airport hotel you can book by the hour but never used it
http://www.bangkokairportonline.com/node/20
Good book, a laptop with a film and have a sit down meal and the time soon goes. Space that out with some shop browsing.
