Up at the top-right you can see some ElkBuffBars, and there's a soulstone timer at the right under the zone map. The minimap has been Sexy-fied a little bit, and below that you can see the zone name, the framerate, the current coordinates, and a percentage that I'm going to guess is durability.
Finally, off to the left is Recount, done up to be mostly transparent. Based on the addon list, a lot more stuff (scrolling combat text, Omen, etc.) will pop up when it's needed.
Here's what Metzerott had to say about it:
My UI is designed around the concept of "Just in time information." I tried to make as many addons as possible only appear to give me information when they have information to present. When the addon is not presenting information it goes away or goes as transparent as possible. Some addons, such as SexyMap, look cool and get to stay at a more visible alpha. This is a very beautiful game. Many people have spent many years building it, I don't like things obstructing my view of it. This UI is made for 1680x1050 resolution.
Just-in-time, indeed. I may draw some features from this for my own UI, in fact. The circular unit frames are not for me, I'm not a fan of the font he's using, and I like to see bars as well as numbers for health/mana, but everyone has their own preferences. I do like how there are no action bars showing; at a guess, this player has most of his core keybindings memorized, and the rest are on oPie menus.
List of mods used:
Ara_Broker_XP: ASCII experience display LibDataBroker plugin
picoEXP: Another LDB XP plugin
Fortress: Displays LDB plugin data
MakeRocketGoNow: LDB quick launcher (to be honest, I don't really understand what this one does)
picoFPS: LDB framerate display
StatBlock_Coords: LDB coordinate display
tekKompare: Always-on equipment comparison tooltips, and hover tooltips on linked items
tekJunkSeller: Automatically vendors grey items
tekErr: Lua error manager
tekDebug: Addon debugging utility
tekability: Durability on the paperdoll screen and as an LDB plugin
oUF: Unit frame framework
oUF_Metzerott: Circular unit frames for oUF, made custom for this UI.
oUF_CombatFeedback: Adds combat feedback to oUF frames (text for events like "dodge" or taking damage)
oUF_Banzai: Aggro coloring for oUF frames
OPie: Radial action menus
OmniCC: Cooldown counts on everything
FluidFrames: Move any frame
MikScrollingBattleText: Scrolling combat text
Bartender4: Action bars
ElkBuffBars: Bars for buffs and debuffs
Omen: Threat meter
Recount: Damage meter
_Cursor: Customizable cursor trails
ArkInventory: Flexible inventory display
GoGoMount: One-key mounting
Cellular: Instant-messenger style whisper chat mod
Forte: I have no idea what this mod is
PowerAuras: Customizable indicators for just about anything
SexyMap: "Minimap awesomification"
Blipstick: Reskins minimap blips ("!", "?", etc.)
ButtonFacade: Support for skinning action buttons
ButtonFacade_Metz: Custom button skins
DKIRunes: Rune and disease display for Death Knights
SellOMatic: Automatically vendor whatever items you want, not just grey ones
Postal: Enhanced mailbox support
Portfolio: Library for addon options
Deadly Boss Mods: Need-to-know information about bosses
Butsu: Slick loot frame
Prat 3.0: Chat enhancements
If you like it, you can download it from WoWInterface. And if you don't like it, and think you can do better, I invite you to submit your own UI to firstname.lastname@example.org.