For $30 you are getting YEARS worth of gameplay. I have never played the storyline in ArmA or Queen's Gambit - there is so much fun to be had creating your own missions in the kickass editor, or going online and playing Evolution!
ArmA has no peer. Like Oblivion, with the addition of mods, it only increases its lead.
If there was a way to deliver COD4 graphics at the scale that ArmA delivers, it would be incredible. But you could probably fit several hundred COD4 maps in an ArmA island.
At least several hundred - plus all of the vehicles.