(For a complete list of what’s new and changed in After Effects CC (12.0), see this page.)
Beginning in After Effects CC (12.0), you can snap layer features to one another while dragging in the Composition panel.
Josh Weiss has a great video demonstrating the new snapping features on the reTooled website.
To snap features to one other, Ctrl+drag (Windows) or Command+drag (Mac OS) the layer in the Composition panel.
When you begin the drag operation, the layer feature nearest the mouse pointer at the time of the initial click becomes the snapping feature. For 2D layers, this feature can be the anchor point, the center of the layer, a layer corner, the layer handle at the midpoint of a layer edge, or any point on a mask path (not just mask vertices). For 3D layers, the snapping feature can also be the center of any face or the center of the 3D volume.
When you drag the layer near other layers, various features on the other layers will become highlighted, indicating that the snapping feature will snap to the highlighted feature of the other layer if you release the mouse button. The feature snapped to can be the anchor point, the center of the layer, a layer corner, the layer handle at the midpoint of a layer edge, or any point on a mask path (not just mask vertices). For 3D layers, the feature snapped to can also be the center of any face or the center of the 3D volume. Note that the feature snapped to can be a feature of a sublayer within a nested composition or text layer with per-character 3D enabled.
The determination of what features of other layers are nearest to the snapping point is made in screen space (i.e., according to the current view), but the snapping itself occurs in composition space. This means that the snapping of one 3D layer to another can cause a drastic shift in layer position.
By default, invoking this snapping behavior requires holding the Command (Mac OS) or Ctrl (Windows) key while dragging. To make the snapping behavior occur without the modifier key, select the Snapping option in the Tools panel. With this option selected, holding the Command (Mac OS) or Ctrl (Windows) key while dragging temporarily disables auto-snapping.
Layers must be visible to be snapped to.
3D layers can be snapped to 3D layers. 2D layers can be snapped to 2D layers.
You can also snap the anchor point of a layer to features of that same layer using the Pan Behind (Anchor Point) tool.