I've recently noticed that JetBlue's JFK-SEA flights have been doing well. I quick look at their last minute fares on jetblue.com shows that their flights go out pretty full (I would guess above 80% LF...I'm sure a B6 employee could give the real figure), but they only have one flight in/out of the market.
It seems that SEA really isn't a hub for any airline except AS, and AS only has very limited service to the East Coast. My question is: Why doesn't B6 expand their SEA operations? I bet they could run a 2nd JFK successfully, and can even perform well to BOS, IAD, and FLL. Is there not enough gate space or something? It looks like SEA is a bit of an underserved market - it's a huge city and if more seats are available, I would think that they'd be able to be filled up easily.