1) Valve is known for releasing massively awesome games with long times inbetween for fans to savor the awesomness. It's ridiculously uncharacteristic to release a sequel that isn't basically a completely different game than the original, whereas this is basically Left 4 Dead With More Stuff. Because of this, the first game is rendered obsolete.
2) All four series you listed have come under fire for having yearly sequels that are more of the same at some point (CoD seems to have rectified this to a degree, though). Just sayin'.
3) Nobody's mad because L4D2 will improve on L4D1, or because we're not getting it for free. Everyone's mad because this is a transparent moneygrab by Valve, the guys who doubled the amount of TF2 content since its release for free. They promised us a continually evolving and changing game, but the extent of this is a Survival mode with only one new map, and an SDK thingy that only %40 percent of players will actually use. To put this in perspective, it's like Valve making Team Fortresses 3, 4, 5 and 6, all of which are the same game as the first, just with more class-specific weapons and some new maps each game.
4) Just for the record, I'm probably going to buy L4D2 anyway, and I'm not jumping on the ridiculous VALVE IS DEAD LETS BOYCOTT ALL THEIR STUFF bandwagon.