Fortunately, there are a few good addons out there which can fill in this gap, and help you utilize your new focus power to the utmost of your creativity.
First and foremost of these is a lovely unit frames addon, "ag-UnitFrames." It allows you to show, arrange, and customize any unit frame, including its own special focus frame, which you can place anywhere on your screen, according to your own needs. Keeping an eye on the 3D portrait of the focused polymorph target, in the mage's example, would allow him or her to know right away if polymorph has broken and needs to be recast. Additionally, a tank could use this new focus frame to keep a close eye on the main healer's health to see if a monster might be attacking him, or if some monster's cleave or AoE ability is still reaching him. With the "target-of-focus" frame enabled, the tank could also keep track of the healer's target, and get a better idea of when heals are incoming for her or for someone else instead. A healer could do the same thing in reverse, to keep track of the tank's health and target, and feel reassured when the "target-of-focus" frame shows the tank changing her target in order to pick up a monster attacking him. All the damage-dealers in your group with this addon could set the focus to most experienced damage player, and immediately see when that player changes targets, so that they can assist and attack the same enemy at the same time.
If you really like the standard blizzard unit frames, or if you just don't want to deal with the hassle of setting up the new ones, there are other focus frame options for you as well. FocusFrame (also pictured above) provides you with focus and target-of-focus frames that blend right into the default Blizzard style, while Hadar's Focus Frame goes for its own, more box-like style.
Have you put the focus feature to an especially creative use? Please share your ideas with us in a comment below.