I made a rough draft of a map the other night, then got the chance to use the PC version. I'm picking up the PS3 version in an hour here, but here are a few things I've learned.
DO Save often
DO play with the textures, but the version I used only allowed 4 textures max. to paint
DO smooth the edges after making rivers. You should make sure anyone falling in the water can get back out just as easy.
DO test your maps for movement ease. Hills that can be climbed, ramps that can be traversed, bridges, vehicles, roads, etc.
DO think of it as a world and not just a level. It can still be fun to play when designed with realism in mind.
DON'T click yes when asked to save before creating a New map, it will save over your map with a clean slate. It happened to me and I was creating for 7 hours straight. I was done, decided to check out a premade layout, and boom, it saved over my work. Gone forever.
DO experiment with structure position and overall verticallity. You can place buildings halfway underground or underwater and the editor won't disagree. A flooded city or buried location could be fun if done right.
DON'T overcalculate the size of the map or play area. I made a "small" map, but it easily fit 10 locations big enough for 16 players. I had a small camp, a small town, a fort, a bridge covering two rivers, a small settlement, a helipad, 3 cliffsides, another bridge, two mountains, a swamp, a four-pronged river...you get the idea.
That's it for now, I'll post when I mess around with the PS3 map editor and find out if I am able to transfer PC maps onto it, which would be useful.
By the way, for those wondering, I did only download it for the map editor. I had full intentions to purchase it upon release. My PC couldn't even run the PC game itself even if I wanted to play it on it....

thanks dude , it seems really helpful