There are a few options in the user interface settings that I consider essential. There are a few others that are just nice. I thought it'd be useful to go through some and pass on a few tips to help out other WoW players.
The interface in WoW refers to all the stuff around the edges of your screen and whatnot. Interface options are things that change the way you interact with that stuff. First off, to get in to the interface options just hit escape and click the button that says Interface Options, it's shown below (throughout the post, click any of the thumbnails for a bigger image).
As of this writing (things may change with future patches), there are two tabs on the screen that shows up. I'm not going through every option, just a couple I think are useful. On the Basic tab under the Camera setting one item is called Water Collision. This option decides whether you can see above the water when your character is below water.
Put another way, when this option is on the camera has to stay with your character and goes under water when your character is submerged. The default setting is on. This is a nuisance if you want to see, for example, what's on the shore but don't want to put your head up. This can be a pain with the Druid swim form. Here are a couple shots of one of my characters swimming around under water.
Once I turned off the water collision option I can take screenshots like these.
Trade windows can be used to interrupt you when you're in combat or disrupt you in other ways. By jerks. Turn the Block Trade option on to put a stop to it, but don't forget to turn it off before trying to trade or get an enchant done.
Now on to the Advanced tab of the Interface Options. There's a setting here that lets you show the target of your target. It's under the Raid & Party section but it applies to every target - in your group or not.
With this turned on you can see when an enemy goes from targetting your pet to targetting you. You could also use it to target someone in your group in a Battleground so you can easily concentrate on the same foe. Here's what it looks like with Show Target of Target set to Always. The small picture below and right of the person you have targetted is the person they have targetted.
Targeting the target of your target is called Assisting. Press
F to Assist and your target's target will become your target.
Finally I like the Floating Text feature that was added in a recent patch. When you do damage or get damaged the number of points is shown right on top your character and drifts upward then dissapears. Other effects can be shown in the same way including heals, periodic damage, dodges, buffs and when most effects proc.
The individual items can be turned on and off but I like to have most of them on for now.