I'd read somewhere that the actor doing the voice-over not only had to be in that day's episode, but their character couldn't be in the opening scene (though this rule was bent a few times, particularly with Alexandra Moltke). Grayson gets my vote for "best voice-overs." Whenever I heard her distinctive husky tones at the beginning, I thought, "Cool! Doctor Hoffman's in today's show, so something really juicy's bound to happen."
As for the text of the voice-overs, they're tough to write on a daily basis (basing this observation on some fan-fiction I've written in the past). Common words in the intro included "terror," "death," "fear," "darkness," and "the great house of Collinwood."
Imagine starting your average day with an obligatory "voice over." Love it! Try it.