If you simply add up the published max TDP ratings for each part in the G74 (CPU, GPU, chipset wlan, screen, HDDs, fans, etc) you get a few watts shy of 150W, and thats not even counting the motherboard electronics (VRMs, RAM, etc, likely another 35W or so)! This is why it is not recommended to be charging your battery while gaming: the adaptor power supply simply is not rated for that much draw! ASUS cut it really fine by delivering the G74 with a 150W supply.
ASUS markets other adaptors at 180W and a 230W brick with the right voltages (see
http://us.estore.asus.com/index.php?l=product_detail&p=4617 for eg)
that would be more efficient, deliver cleaner power (less ripple) at 150W and operate cooler, but the downside is they are bigger and less portable. But for a parked G74 they would be perfect: I myself am considering the 230W unit for my office desk.
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G74SX-CST1-CBIL, i7 2630QM 2GHz
32GB DDR3 RAM @1333MHz
GTX560M 3GB DDR5 (192 bit)
17.3" LED 1920x1080
Sentelic TP, BIOS 203
Debian Linux Wheezy (Testing) Kernel 3.2, NVIDIA 295.40