I think the best part of my time playing WoW has been designing user interfaces (UI), and I thought I would share my concept with other Holy Paladins. I really suggest you read this post, and I hope it will inspire you to make a better UI yourself that fits you. You may believe that this post deserves to be in the UI & Macros section, but this concept is more focused upon Holy Paladins, and I don't want to bother with stuff like "What about my totems?" or "I need more bars for my abilities!".
I have always sought 3 simple goals with my interface: simplicity, information and consistency. This should grant the player the best way to control his or hers unit (which an UI is meant for). I use this golden rule: If it's not used in combat, then hide or remove it.
UNIT FRAMES
These are the main frames: player frame, target frame, target's target frame and focus frame (see raid frames below). Player and target frame should be consistent in design (size, colours etc.), but the information different. Player frame should show current health and mana with % too, but don't show the character name. Target frame needs only to show current health with %, level and name. Target's target isn't that important as frame as target, so it's only showing name and health % on mine (with no frames). Keep the focus frame near middle screen or raid frames, so you can give it a quick glance.
ACTION BARS
I am using a very different way to sort my action bars, and it may sound complicated, but really it isn't.
I have two bars that's hidden and only become visible when you hover a mouse over them: aura bar and bar for mounts, professions, buffs (seals and blessings) etc. It's located below my visible bars.
I have one entirely invisible bar that contains all my key bound abilities (the button beside "1": Judgement, 1: Holy Shock, 2: Flash of Light, 3: Holy Light, 4: Divine Light, 5: Word of Glory, R: Light of Dawn, T: Holy Radiance). I don't need to see these abilities, because I am not clicking on them and they have no cooldown (except for three of them).
I have three bars that's shown. One is bigger than the others, and it shows me my key bound abilities that has a cooldown (Holy Shock, Holy Radiance, Judgement). Two that shows other abilities that's used inside combat: Hand of..., Divine Favor, Divine Plea, Lay on Hands etc.
RAID FRAMES
These are the healer's focus, and therefor they need to be centralized and informative. I use Grid, and with some edits it'll work just like Blizzard's in-game one. It show icons when raid members have debuffs that either are removable or are important to be aware of. They also show a pink mark on the person with Beacon of Light active.
CHAT BOXES
I prefer having two: one for human made chat (say, party, guild, raid, channel, battleground etc.) and one for data (loot, honor, professions, boss quotes, battleground activity). I think this should be baseline of how chat works, so REAL chat is uninterrupted. My tooltip hovers above my data chat.
BUFFS AND DEBUFFS
I think buffs and debuffs should be kept small besides the minimap, because the important buffs are shown through Power Auras Classic (see below) and important debuffs are shown through Grid (where you eyes are anyway). I have never shown target buffs and debuffs, because the only buffs and debuffs I need to be aware of is of mine raid members, and I can already see the important ones through Grid. But it does get annoying when you're in town, and you want to know the new mounts name. Target buffs and debuffs are overrated. ;P
POWER AURAS CLASSIC
Blizzard might have added the new cool effects for when you can use Holy Shock twice, or your next cast is faster, but I still like Power Auras Classic to be my notification for my Holy Power, when Beacon of Light is missing or show the duration of Divine Plea. This addon is awesome to highlight important stuff in the correct way (instead of looking into your buffs and spend time searching for Divine Plea's duration left).
COMBAT TEXT
I remember when I was a Druid in WotLK and Blizzard decided to make a change to combat text where it would show overhealing too. My screen was filled with green text, because I threw Rejuvenations here and there. So I installed a combat text addon and I made it 2 sections: incoming and outgoing. Incoming would show everything my character receive (healing and damage) while outgoing would show everything my character performs (healing and damage). It won't appear over the head of character, but just in a fixed spot. Again, I don't find it important if I healed 10k or 11k this time with Holy Light. Therefor I keep it small and simple. You can see an example here of a person using my previous UI (look at 0:40). Notice the combat text.
OTHER
Notice how I have important UI elements near each other (action bars, raid frames, unit frames, cast bar etc.), and my less important UI elements away (minimap, tooltip, chat, buffs and debuffs). I often see players playig around their UI moving different UI elements around, and even though the UI is pretty when minimap is in bot middle (example) it servers no purpose there and only makes the UI become a mess. Keep the central screen for important stuff, and cornors for less important.
This was a bit about my interface. I hope writing this will inspire you to make your's better, and that it will help improve yourself as a player. ;P