What do they need to know?

How to get notifications. (Gina or SteemWorld)
Posts only earn for seven days.
The little number at the bottom of other people's posts contains no information.
If your voting power is 100% you're wasting money.
People love to complain about nonsensical things here just as much as they do anywhere else.

What will help most stay?

Enough early rewards to justify learning how to earn more.
Enough social interaction to convince them there's somewhere to build to.
Good examples of socially-successful users who've put in the time.

What is the thing that frustrates new users the most?

The interface. Not understanding how to get notifications easily. The confusing nature of a system that runs on three related currencies.

What can a community do for them? Can it even do something?

Those of us with SP can do something about keeping a base level of rewards for posters we want to keep.
All of us can interact with their posts on a meaningful level and give them something to think about above spam.

How do we level the playing field? Can we even do it?

We can't, but we can turn some of its non-level features to our particular advantage.