It depends on the target audience, at least for me.
Publicly offered story arcs - stuff that basically begins with a new +event accepting all comers - are the easiest, I think, because usually I want them accessible to all comers and I don't have to motivate them too much. All I do is offer a general sense of what it's about ("social", "exploration", etc), the expected risk level, and maybe some very basic IC requirements for joining ("you need to be a Vampire or ghoul in good standing with the Praxis").
Stuff I run for my friends also don't need to include spoilers because I get to talk to these guys ahead of time; I tend to have a decent idea of where they're at, what their characters are about and what they want to do on an OOC level. The tricky part here is to not mold the story too much after those things or in a way they are then spoiling themselves; if they're talking a lot about wanting to go to the Hedge and acquire a magical sword, I need to be careful to either not do quite that, or to twist their expectations in some way.
Either way what's a larger risk - for MU* - is to make people feel like they're in control, nor the story. It's extremely common for STs to place PrPs on rails which is, after all, the ultimate spoiler if you can see where everything's going and you can't even have an effect on it other than to go through the motions. I feel that's the greater risk.
But as for the content... it's a tricky one. My publicly offered scenes tend to be relatively PG13, or at least with no more than one finds in a Hollywood film (that's my standard at least). Little of it applies to stuff I throw to more specific audiences though since they theoretically trust me, and I know them at least somewhat, so I can avoid pitfalls (or communication is open and they know they can tell me if something's heading into hot waters).
