I think the big thing is that, as most of you point out, an American wearing the flag suit. If it were Steve Rogers in the suit we'd still be miffed, but it's worse when it's aguy who was "used" to murder hundreds of people, many Canadians, is given the honour and distinction of wearing the flag suit for no good reason.
He has not shown any 'hero' ability, any desire to be a hero, no desire to protect Canada or anyone else, just remorse over what he was engaged in. He has no ties to heroism, brains, fighting ability, Canada, or the team. He is a guy with powers that allow him to be manipulated, so he gets a flag suit designed to help him use his powers, I guess to prevent his lack of usefulness from killing more people, so that he can be a hero.
Does he want to be a hero? Join OF? Become Canadian? Is he Canadian? Mike Oeming may have something up his sleeve to exaplin why this weak-willed accomplice murderer gets a free pass and gets trusted to join a foreign country's team and figure out what he can do not to kill anyone else, let alone save anyone.
His power is mutant energy absorption. Oeming will be sure to retconn that so that he can absorb other energy forms and actually use the energy. Likely that is what Richard's suit is for, to prevent Mailman from killing people by accident anymore and to actually have a way to use the energy he sucks in
So for me the character needs one or two things: a reason to actually be in OF as opposed to Great Lakes Avengers, and/or a name not tied to Guardian, Vindicator, or Weapon Alpha. Maybe he'll be Weapon Omega II.
It's clearly up to Mike Oeming to make this character more than the cypher he is now. After issue #2 we'll either know Mailman is a worthy Canadian, or an embarrassment.