As Deraldin said, you need 100 ammo to fire. Also, you have to hold the fire button down for quite awhile (about 5-10 seconds). This may sound extreme, but this will pretty much wipe out a room.
Probably not the best use of ammo, but it did feel suitably epic.
Not too crazy about the changes in the equipment system - I can't help but think that eventual DLC is going to add quite a bit in the way of new guns and armor (and I really hope it does, I was using the basic N7 armor without a helmet and with just the ammo-boosting arm for the whole game, in spite of having access to 3 dlc armors). Also annoyed I missed out on the
.
There were some really amazing cues on the soundtrack again, but I guess it was shooting for a more subdued overall impression and it didn't work quite as well for me - there were some really good actiony ones in the first game, especially love the eden prime and feros action tracks...