Addon List (Updated WOD 6.0):Auras: TellMeWhen!!! My super replacement for Power Auras and Weak Auras, these 2 addons didn't work well for me at MOP release (auras disappearing out the config, either not working in combat or making it so you could not edit them again) so I had to swap and I never looked back. The awesome project manager Cybeloras is really helpful too!
To the left, right and under my toon I have icons show if an important attack is off cooldown. To the left (and sometimes right) of the bottom icon are important cooldowns available, some only show on bosses and in PVP. Then just above my action bars I have a set of 4 icons, 2 on each side to show important DOTs, buffs and debuffs, with on some toons extra smaller icons above those if needed (like for a Monk). Some classes also show available AOE attacks mostly on the far right of the bottom icons. Most icons have a range of conditions configured for them so they only show when they are most useful/needed.
Buffs & Cooldowns: Raven & OmniCC.
Bars: Bartender & Masque Renaitre Fade button skin. I use a Razer Naga <3, the left and right action bars use the shift & control modifier buttons. There are two mouseover action bars hidden at the very bottom of the screen. Which I use along with Opie-addon rings for less used macros and can be used for shortcuts to consumables, mounts, equipment sets, etc.
Chat: Basicchatmods. Left window: whispers, party, raid, general, Dbm, boss emotes. Right window: guild, trade, loot, npc emotes, xp, rep, profs, etc. Makes it impossible to miss anything important.
Combat Text: Mikscrollingbattletext.
Dps meter: Skada damage meter.
Frames: Pitbull for unit and party frames.
Garrisons: Bulkie, Bulkorder, Garrison Mission Manager, Master Plan.
Nameplates: Tidyplates + Threatplates.
Others: Adibags, Altoholic, Atlasloot Enhanced, ClcEclpipse, _Cursor, Collectinator, DBM, Deadly PVP Mods, Eclipse Bar Plus, Energywatch Reborn, Grid, GTFO, Handynotes Draenor Treasures, HHTD, Killshot, Leatrix Plus, Moveanything, NPCscan, Oneclickenchantscroll, Opie, Postal, Sexymap, SharedMediaAdditionalFonts, Simple iLevel, SmartRes2, UmberRunes.
No longer using but worth a mention if it might help you: Addon control panel, Afterlife Crowd Control, Eclipsor, Fonter, Gatherer, Gogomount, MagicRunes, Tauntmaster.
You can find most of these addons on Curse, if you can't find it there it's on Wowinterface.
From old to new
The 3rd oldest I moved the unit frames to the left and right of my toon, and made my own party frames with Pitbull. Got Tiptac for my tooltips. The Power Auras started surrounding my toon, used Raven for buff & debuff management, created a 2nd chatbox on the right and started experimenting with fonts.
My second to last UI - I got rid of Icehud and the cartoony Harlequin font except for Combat Text Crits (using Exprswyfree now). Created a template of TellMeWhen Icons (instead of Power Auras) for all my classes and specs to build around and changed my Castbar addon to Gnosis.
10 man raid
40 man Sha raid
My current UI which I'm quite happy with, quite clean out of combat, the last addition is Pitbull raid frames which I needed cause replacing fonter gave the blizz raid frames back their standard font which I really don't like.
10 man raid- On ten man the debuffs are very noticeable too
5 man party LFD
Current UI - Out of Combat