The "Game of the Year" edition of Left4Dead is a bit confusing, and you might call it an outright misnomer.
If we're talking about the Xbox 360 edition, it comes with just the Survival mode, which was free DLC anyhow - but it's on the disc, as opposed to something you've got to download from Xbox Live. If you want the other campaigns, Crash Course and The Sacrifice, you're going to have to pony up 560 Microsoft points (~$7) each (or wait for a sale/price drop sometime in the future)
If you're talking about the PC edition, it comes with all the DLC campaigns, but again, that was free DLC for PC users. You'll get everything via Steam when you install it.
Effectively there's very little difference between the "Standard" and "Game of the Year" editions on both platforms.