They start flying from BWI to Reykjavik five days a week in May. Those fares start at $179 one way.
Connections are available on WOW to Berlin Schönefeld, Copenhagen, Dublin, London Gatwick, and Paris Charles de Gaulle.
They're a low-fare airline, but they are ticketing the through connection, so they'll probably take care of a problem if you're connecting at Keflavik airport.
The seat pitch on their A321s is 30". According to routehappy, this is average.
(Edit: corrected number of days of week for Boston to Reykjavik flights.)