I think the best CFL team could WIN against an NFL team if they played by all the CFL rules. (We'll play with our Canadians, but let the NFL team off the hook for that.)So, CFL field, 3 downs, and most importantly, ROSTER SIZE.
Some NFL teams have 3 kickers dressed. One for kickoffs, one to punt, and one to kick field goals. The NFL active roster has about 10 more players than the CFL's. The NFL features lots of specialist players. Such as DT's and LB's that only play run downs (or pass downs). And they have "return only" guys. The starters in the NFL don't play special teams, but they'd have to in a CFL setting (again, due to the smaller roster). The NFL guys would get worn out from playing so much more of the game because there would be less substituting. (I'm thinking mostly of the big men from NFL teams, receivers and corners wouldn't have a problem adjusting.)
And let's not forget about the rules that they wouldn't understand! No yards. The rouge. I don't know if they'd know how to use the 12th man on the field.
The dimensions of the field would freak them out. The 55 yard line?!? Or all the room on returns. Or Quarterbacks having to adjust their throws on the wide side out pattern.
These are just the first few thoughts that popped into my head, but I'm sure there's plenty more.
Oh right, another MAJOR factor: the 20 second time clock! They would take a ton of delay of game penalties.
And no 3 timeouts per half!
...lining up a yard off the ball on defence
...the strategy changes they'd have to make with only 3 downs
...and the QB wouldn't have a head set in his helmet
...how would they defend the much larger end zone?
...what would they think of the goal posts at the front of the end zone?
...and they wouldn't know about the rule where the kicker can recover his own punt, I can just see the NFL team waiting for the CFL team to down the ball and the CFL punter recovering the ball.
...on a ball going out of bounds the CFL team only needs to touch it last to gain possession
There are so many subtle differences in the rules that the NFL team would scream bloody murder that the refs were conspiring against them.