I'm more of the mind that they shouldn't be for things you HAVE to do to complete the game, though they can be for completing the game as well. They should either be something that is above and beyond, or something unusual you do. I don't want to get an achievement for beating the tutorial level, since even a blind retarded monkey could do that in most games, but giving an achievement for killing a tank with a frying pan (L4D2; ok, it was any melee weapon) is a fun and hilarious achievement. I also like ones that are a good trick to learn anyway: punching a hunter as he pounces and killing a witch with one shot are good examples of useful skills turned into achievements (L4D).