Addons!

I came late to the concept of addons. My first addon - I believe it was MapCoords - was installed when my first character was late 60s. When I made my Paladin, I levelled to 80 simply using that one addon; when I hit 80, I deleted it.

I tanked Naxx clears and other 3.0 raid content for a number of weeks without any addons, then finally installed Deadly Boss Mods. I tanked Naxx clears for several more weeks with just DBM before deciding to completely rebuild my User Interface. A customised UI, depending on just what you want to do with it, can require a large number of addons.

Here is a list of all the addons I have installed:
All Stats
Atlasloot Enhanced
Auctionator
Bartender 4
BlizzMove
Deadly Boss Mods
eCastingBar
Grid
LunarSphere
Message Redirect
MikScrollingBattleText
Omen Threat Meter
PallyPower
Pitbull Unit Frames 3.0
Power Auras Classic
Prat 3.0
Recount
SatrinaBuffFrame
SexyMap
SmartRes
SmoothDurability
Taunt Tattle

The ones in bold are the most important ones.

And, if you're interested, here's what my UI looks like currently:

Click it!

There are some things that it's difficult to get a screenshot of, and some things that simply aren't shown because they weren't relevant at the time I took the screenshot. The most important aspects of my UI are visible though.