Hi, I'm ShadowfangXIII and this is my first guide! W00t! I usually play mid/support characters, but I actually came up with this build the first time I went top with Diana and wrecked. I'm not very experienced at writing things like this, so forgive me for not putting much detail. I'll try to list everything I know that is within my experience.

Pros

Will wreck face late game
Can solo Ancient Golem at level 6 without taking almost any damage
Dat shield AOE
No CD on ult if used with Moonlight
High mobility
Fun to harass with when enemies tower hug
Usually easy to get first blood with


Cons

Early game is okay
Stuns and silences ruin your day if they disrupt your Q/R combo
Enemy Jayce (if mid) or Jax (if top) :/

Runes
Diana can be very mana starved throughout game if you mid or solo top because of her burst. These runes are picked to give her a better sustain and maximize your late game damage output.

Masteries
In order to be really tanky for your teamfights, I suggest this Mastery setup rather than putting points in utility for sustain since we used runes for that. You want your defensive presence to be formidable, especially if you're going against somebody who also has high burst or CC.

Summoner Spells
Flash will maximize your mobility for ignite to clench the kill. Teleport and revive will work for teamfights past laning phase if they target you first (they have to shut down your killing spree somehow :D).

April 1, 2013 - guide released (no joke LOL)
April 24, 2013 - organized item order better and redid skill selection order
May 23, 2013 - I've gone on League hiatus to find out Diana has been nerfed. Will update when I find out if this build still works or not.