Turning off selection highlights
When you select a complex object with a lot of nodes, you might find all those nodes distracting. Fortunately, you can hide the nodes by disabling the Show selected object points option in the Display page of the Options dialog box. When this option is off, selecting an object displays its centerline path and the rotation and scaling handles, but not its nodes. Note that you can still select and edit the nodes using the Direct Selection tool and Lasso Selection tool if you know where to click. Once you've selected specific nodes, they do appear.
If you want to hide both the nodes and the centerline path to get an entirely unobstructed view of your selected objects, you can pick Edges from the Show submenu under the View menu (or press CTRL+H). The object(s) remain selected, but the only indication is the rotate and scale handles around the object's bounding box. You can show the highlights again by clicking anywhere on the object(s).
The same object showing Edges (left) and with Edges disabled (right)
Note that you can also control whether Expression Design shows you the eight rotate and scale handles around selected objects with the Show resize handles option in the Display page of the Options dialog box.