If it had co-op, it would be amazingly awesome beyond belief. The campaign is awesome as it is, but if you could play it together with someone... :0

Also, Claymore Heavy Shotgun is way cool. Just get up close and KABOOM. Takes down an armored krogan in two shots, maybe even a single shot if you're at point blank range. Don't know for sure about that, though. Low rate of fire, but very high damage if all shots connect. Delicious. :U