I made a thread a while back about gameplay vs. storyline. As Faw said a good plot can help encourage people to play more and it increases the stakes, creating suspense and satisfaction.

However, a plot is nothing without solid gameplay and scenery to help back it up. You don't necessarily need a good story, though, a single player pack that can stand on gameplay alone is fine.

As for multiplayer levels, layouts are always key. I have never been a master at creating good multiplayer layouts, though. Just study aspects of popular levels and try to see what about them fosters good gameplay and whatnot. Flow is usually very important, it's generally understood that you should be able to create routes through the level without ever hitting a dead end. There are a lot of nuances that lie between an average level and one fun to play in. Eye candy is also a plus for multiplayer, but it's not as necessary and sometimes overdoing it makes the level annoying.

It's also always good to take some risks in implementing new ideas. Quality innovation is always appreciated, and it helps better other creators by forcing them to reconsider some unspoken rules that may not necessarily be the best. It's a risk because most of the time a wild new idea won't be very well realized until you've pinpointed the aspects that need to be changed.

rambling done