09 Aug 10, 11:10PM
Probably has something to do with your map having too many wide open areas, there is a new map-density check in 1.1 that checks the basic construction of a map to decide whether or not it is of decent enough quality for multi-player use.
Try adding more solids and making the large areas of your map less large - both in width, height.
Try adding more solids and making the large areas of your map less large - both in width, height.