(Taking a cue from DMMike's evil post.)
Any good alignment, within the broad range of goodness from too-good-to-be-real to just-good-enough-to-be-called-good can be hard to play in a setting where killing is par for the course. And fantasy settings include a lot of violence/combat, where death is a regular occurrence (for the monsters, at least).

What makes playing a good character fun?
Is it just roleplaying that make believe ideal of goodness?
Where do you draw the line at where good starts in your game?