A Well thoughtout User Interface will help give you a edge in combat. Information is priceless in guessing what the enemy plans to use against you, and thereby giving you a way to counter it.
Example UI

ADDON List(Shown)
- BATTLE GROUND TARGETS - Gladius for BGs
- Quartz Cast Bar - resizable highly visable cast bars for landing interupts
- Bartender 4 - Action Bar addon that allows you to resize/positon your bars
- Nameplate ADDON - Tidyplates - Threat Plates and Plate Buffs - Shows Castbars and CDs/CC/DOTs etc on enemy nameplates(class colored too for easier target recoginition.
- INTERUPT BAR - tracks enemy players CDs on interupts to help you juke and not get locked out.
- SHADOWED UNIT FRAMES - customizable replacement unit frames for the bliz defaults
ADDON List(Not Shown)
- Auto Repair - cause i always forget to
- Tricks - Announces when you have used CDs and successfully interupted something
- GLADIATORLOSSA - Announces enemy CD uses via in game sound - requires quite a bit of settup to be usefull in BGs othr wise you will get spammed(most overpowered addon to date!)
- Gladiusex - Arena Addon - Enough said...
- Healers Have to Die - Places a Red Cross above enemy healers in BGs, Usefull for quickly finding enemy heals to cc/blow up
- Lose Control - places visual icons next to unit frames for friendly(or if settup arena enemy 1-5) when cc'd
- MIK;s Scrolling Battle Text - condenses battle text info into a customizable notification areas. Can change what specific spells/events trigger an elert
- Weak Auras 2 - Able to place visual and audio trigers to events on screen(i.e. CDs on spells, cc, etc)
- PRAT - Chat addon
- WIM - Wow Instant Messenger - Yahoo/Aim like addon for /tells - usefull for setting up groups