What stands out most distinctly when previewing a Call of Duty game is the severity with which the subject matter is taken. Sorry, both subject matters. Because FPS games are subject to two very important matters: being games, and being about something. Within the development of all four Call of Duty titles, whether by Treyarch or Infinity Ward [or Spark, don't forget - Ed], both sides are taken so seriously that you begin to realise why the finished games are quite so affecting.
We might occasionally bemoan the incessant glut of World War II games, but Gott im Himmel, they sell. In fact, Activision shifts so many of the blessed things every year that it has two developers working on the Call of Duty franchise at all times. So while Infinity Ward (no doubt) beavers away on its next PC-related project, Treyarch is conjuring Call of Duty 3 for all the various console platforms, including Xbox 360, PS3, Wii, PS2 and even the dear old Xbox.