I hate missable achievements. I like to go through a game normally. I don't want to follow a guide to make sure that I get the missable achievements. If I miss the achievement, I miss the achievement. I'm not going to replay a game from start to finish just to pick up achievements I missed.
Definition for those that don't know what those are: http://www.giantbomb.com/missable-achievements/92-4261/
As for ideal achievement design, I think there should be some "right on schedule" achievement (say one for each chapter, act, stage or whatever and for beating the campaign) but the majority of the achievements should either be a challenge (without being a tedious grind. ie. 100,000 kills or collect X amount of Y object or open every treasure chest in the game. Gears of War, Crackdown and Lost Odyssey, I'm looking at all of you) or encourage different ways of playing the game, rewarding the end-user for experimentation and discovering something neat about the game. I do not like achievements with a grind-heavy sort of nature. Takes the fun out of the game.
That said, I'm not complaining when achievements come easy. lol. The gamerscore is nice. But in principle, in order for achievements to really mean something, the challenge and experimentation/exploration/hidden easter egg factor should come into play.
Ultimately I guess I'd say a mixture of all three, with more emphasis on "why didn't you warn me?" and "I need a strategy guide"