I also wanted to make an early post on the key points of a good UI. I believe being a good healer and having a good UI are strongly linked. It's true that anyone who cares a lot about their class/role will want to tweak their UI for it, but I think healers have a special need for lots of information, and easy ways to act on it.
In fact, that sums up what I think are the two key points of a great UI:
1) Provide information
2) Allow action
This is because everything we do in the game comes down to observing the game world, making decisions, and then carrying them out. And since ideally the decisions should be made by the player (at least the non-trivial ones), that leaves the two areas of providing information, and enabling action, as the functions of a good UI. Good AddOns should help us gather information efficiently, and carry out our desired actions more effectively. This leaves us more time for the crucial decisions themselves.
When it comes to displaying information, there are two issues. First, is all the information you need available? And second, is it displayed in a useful manner? So first of all, does your UI show healing, damage, HP, mana, buffs and debuffs, and everything else you'd want to see? And secondly, is the UI set up so that you can effectively access and use all that information? If we could parse an unlimited amount of info, we could just read our combat logs as they fly by. But we have a limited amount of focus, and so a good UI needs to display relevant information, quickly and succinctly, in a way that's easy to understand.
For example, a good UI should show you if you get a clearcasting proc. Knowing that you can cast a big spell for free is useful information that could affect your spell selection. Now, you could turn on buff notifications in FCT/SCT so you can see that you gained clearcasting. But in combat, will you be able to watch all those buff names fly by, while watching health bars? Perhaps a better idea is to have a mod that pops up a graphic when you get procs you care about. Or plays a sound effect that you can associate with clearcasting. This way the information is there, in a way you can easily observe even while doing other things.
The other side of the UI is how it lets you take action. Once you've made a decision (hopefully quickly and correctly thanks to good information displays), you want to be able to carry it out with a thought. You don't want a multi-step process in order to cast a heal or important buff. You'd like each important action to be achievable through a direct, quick, easy action. Healing or shielding someone or buffing them in combat should only require a click, or a keypress, not a lot of mouse movement to find them, target them, select a spell, and cast it. This is why Emergency Stop switches are large red buttons as opposed to combination locks.
The best example of this for healers is a click-to-cast mod: it's easy to save somebody when you can just click their health bar. But this also applies to making sure all your important spells are assigned to clicks or bound to keys, setting up macros for important tasks, using mods or macros to activate trinkets as opposed to clicking them in your character sheet, and so on.
Overall, in my opinion, the goal of a good UI is for it to display all of the information you need, succinctly, quickly, and parsably, and to allow you to act on that info directly, quickly, easily, and completely.