For me my tricky ones were Marathon, Ninja, Hacker, Steady and Scavenger.

For Marathon my problem was First bloods, Ninja just peeved me right off because I had to play Demo mind you on this prestige I got 8 plants in one round, Steady with the double knife in 5 secs is just random, Scavenger and Hacker i've just gave up going for pro versions and sticking with normal because I dont use grenades or tac grenades (wily pete but cant resuply anyway) and Hacker just takes too long to get Claymore kils and I lose my insert for too long while trying to get them.

So yeah thats basically why I'm hoping its more like MW2.