To get the most out of the game, most folks are using mods.
I use wow.curse.com to download mine, but there are lots of ways to get mods.
When building a UI you should think about the categories of mods first, then start filling in the details.
So:
Action bars - I use bartender
Raid Frames - I use xperl for my target, myself, pets and I use Vuhdo for raid
Boss Warnings - I use Deadly Boss Mods
Threat meter - Omen
Damage meter - I use recount (Skada is a great option)
Chat - I use Wow Instant Messenger
Inventory - I use Bagnon, but lots of folks use ArkInventory
Mail
Professions
Map - I use Sexymap
Questing
Timers (casting timers) - I use quartz for my casters
Class specific - for healing I use Vuhdo
That's what *I* use, what do YOU use?
By the way, check out www.nostockui.com