NL is different than FL in calling raises with 22, which I recall is a losing limit play in most cases that don't involve your blind. I could be wrong about that it has been awhile since I've played fixed limit.

You hit the nail on the head on the reason why small pairs are profitable in a NL game. If you would have called with 44 you would have got his stack and the hand is easy to fold otherwise.

The average rule for calling these raises that most people exercise is 10:1 implied odds. The worse the opponent is the closer I will take bad implied odds, the better the opponent is the higher I like it to be. A good opponent that you can't stack with AA you need him deep enough to try to get 10:1 out of the hand. A bad opponent that you have position on might be raising KTs, and an undercard flop will allow you to take it down uncontested with the best hand or after raising his continuation bet if he is aggressive, the set wasn't even necessary so implied odds matter less.