You can create up to 10 private maps (within the total of 20 MB of private storage), but unlimited public ones.
The Data button in the lower left of the map can be expanded to see the underlying attributes of the dataset being mapped.
The Layers Palette is in the upper right of the GeoCommons window. It shows the layers open, whether they're active on the map, and the which are on top of which. It's somewhat comparable to the Table of Contents in ArcGIS or the Layer dialog in PhotoShop.