1. being extremely easy to use
2. being extremely powerful
a nub cannon, also knows as a n00b cannon, a NUBZ cannon, etc etc is usually something that people bitch about when they get PWNT.
The people who complain usually turn out to be nubs anyways, as l33t players don't fall prey to nub cannons.
PSR sniper rifle in Counter Strike
M16A4 Carbine in Call of Duty 4
SAW in Call of Duty 4
many types of mounted cannons and grenade lunchers