The reason for all forms of media, be it book, film, newspaper article or game, is to tell a story. Even though, say, a newspaper article is there to inform rather than entertain, it still tells you a story about something.
So, I think that for a mission or campaign to be succesful, they need a storyline. Personally, I think CWC lost it a little, because the opening missions were great in the sense that you were one of the grunts, and there was bantering between your mates before the fighting. It was like 'being' Armstrong, and when you feel part of a story that your character plays a role in, it's suceeding. In games like Black Hawk Down, which have no real story, it felt a bit... exclusive. Like your part wasn't important, and that you were just a faceless grunt. I think CWC went this way a little bit once it started chopping and changing, with only Gastovski and Armstrong interesting me. The tankie didn't interest me, and the pilot only interested me when he was shot down, and you had to escape and evade the enemy. It's the little twists that make the differences sometimes.
My mod campaign, Hoffman's Luck (sadly defunct) was based on a story I wrote about a disillusioned British soldier who was part of a unit garrisoning a small island. It had it's fair share of characters (a few of which that were to be playable) but each was important to the plot, since their parts all intertwined. CWC seemed to have been written with only Armstrong and Gastovski in mind, then the others bolted on at the end.
I'd write about Res, but I only got up to the bit where you had to ambush the convoy, it was too hard for me
But, I did like the choices thrown at you, like betraying the rebel, or fighting the Russians, and then having to deal with the consequences of your actions. That was unexpecteldy cool.
So, to summarise:
A mission IS the storyline