I plan to go into detail about the pro's and cons of items/runes/matchups/masteries as more testing is done.

Pro:
Great farmer (reasonably low mana costs)
Great damage
Great survivability
versatile champion (top, mid, jungle, support?)
mobile with Lunar Rush
Great Ganker (post lvl 6)
Amazing kit (not as item dependent)

Con:

No real escape mechanism (other then bursting them down)
Cooldown dependent if built bursty (barely a problem)

The Ap ratio of Crescent Strike is down from .8 to .7
the dmg of Crescent Strike is down from 80, 125, 170, 215, 260 to 70, 110, 150, 190, 230
Health regen starts at 7 +(.75/lvl) down from 9 +(.85/lvl)

I dont think these nerfs will affect her too much. she will still dominate.

The rune build displayed above is geared for an easy matchup against an Ad champion, most likely top lane due to the Greater Seal of Armor. For a mid-lane game, swap the seals for mp5 and g;yphs for Mr.
I say easy because of the mana regen provided from your glyphs; with the low costs of your Crescent Strike you can virtually spam non-stop. This makes last hitting cs extremely easy and does not require you to even go anywhere near the enemy champion if you wish it.
Having said that:
A more defensive or offensive line up can also be beneficial.
mag pen marks, armor seals, mag res glyphs, Ap quints do nicely
or
mag pen marks, armor/mag res seals, ap glyphs/quints will have you doing massive dmg very early on.

Speed Quints are also appealing.

always start with Boots and 3 Health Potion
I then rush a Philosopher's Stone for added sustain and gold. Its a great item to have, especially when you dont have access to blue buff. Try to obtain by 6 mins

If you dont like to build a Philosopher's Stone a Kage's Lucky Pick into Deathfire Grasp the best bursty substitute.

Catalyst the Protector to Rod of Ages try to obtain by 20 mins

Boots are always a hot topic but here is what i think. Diana has solid dmg and survivability. due to the abilities she has. Ionian Boots of Lucidity provide much more dmg and survivability by lowering the cooldowns. I will cover this in the Skill Chaining section, but to chain Crescent Strike, Lunar Rush, Crescent Strike, Lunar Rush, Lunar Rush requires a low Q cooldown.

It's debatable, at this early junction, what is best for which situation. i feel that Rabadon's Deathcap which will up your AP total somewhere around 450-500 will decimate most champions in a matter of seconds.
Rylai's Crystal Scepter Provides more survivability, but packs much less of a punch. the slow is rarely useful IMO due to the fact that with a deathcap they dont have time to run.

After I complete my deathcap i finnish Shurelya's Battlesong giving you that extra burst of speed all assassins love.

Situational Items

Lich Bane is my favorite choice by far. makes you faster and even more bursty. a real menace.
Nashor's Tooth / Malady are AS options, more suited for taking down the jungle/tower/inhibitor a job better done by your ADC. My build is framed on being an assassin: your bursty, not a sustained/tanky oriented mage.
Abyssal Mask and Zhonya's Hourglass do provide you with alot more survivability allowing you to even initiate (pls do not attempt in a 5v5) more often.
Mejai's Soulstealer she kicks so much ***...

Items I don't like: (note: i said I dont like)

Hextech Gunblade, Ginsoo's Rageblade is for sustained dps, not burst
Athene's Unholy Grail is not needed if you have Shurelya's Battlesong
Archangel's Staff meh


My posted build:
Ionian Boots of Lucidity
Shurelya's Battlesong
Rod of Ages
Rabadon's Deathcap
Lich Bane

Is extremely powerful, boasts low cd (34%) and still quite survivable.

Do people still need to be told how important buying wards are?

Whether you are doing good or bad, maxing Crescent Strike is beneficial. cooldown is lowered, dmg is increased, but the mana cost stays low. It allows you to farm quite efficiently from a safe distance.

If you find your getting ganked or dived too often consider maxing Pale Cascade first. It's mana hungry, but well worth it if u survive.

If you feel confident, feel free to get 1 point in all 3 abilities by lvl 3. If your concerned about being zoned put 2 in Q.

I always go Ignite Flash
it has not failed me.

But I quickly find Ignite to lose it's usefulness very early on. I will start using Teleport to (somehow) increase my kills.

Please use flash aggressively ffs.

Diana is extremely versatile, her kit is quite amazing and should never be compared to Akali just because they have similar ultimates...

Q -> R -> W -> R is the "standard" attack combo

Do not always immediately use your R the second time (putting it on Cooldown). It can be a very useful tool to escape (dash to minion) or catch up if they flash.

Don't be afraid to use your R to initiate even if, god forbid, your Q didnt land. Any half decent player would probably move back towards safety as soon are your q lands.
Surprise them:

R -> Q -> W -> E (if they try to run) is still quite potent; the range on your R is just tops. (900)

Your E is a very valuable tool: if you ult into a close group of champions and E, they are all knocked up and pulled towards you. allowing you to land your Q and W on all the champs.

Q -> R -> E -> W -> Q -> R -> R Is probably the most destructive AOE, team disrupting combo you can do.

dont forget to AA as well, your passive is oh-so sweet and deals ALOT of dmg

To land a proper Q try to lead the target. practice if need be.

Be aware of your opponent's attitude. Aggressive? defensive? down right passive? If your facing Katarina she will most likely jump you when she sees you moveing up to land your Q. Be sneaky:

W -> Q -> R ->
Shield first, then unload on her.

If your crazy enought to attempt a 5v5 initiate try something along these lines:

W -> R -> E -> Q ( then pray they are all dead or running, cause you have nothing left for a couple seconds)

To be added with more testing

My rune setup for Jungleing:
Greater Mark of Magic Penetration
Greater Seal of Armor
Greater Glyph of Ability Power
Greater Quintessence of Ability Power

Items:
Regrowth Pendant and 1 Health Potion
Rush Philosopher's Stone
then boots, then follow the build posted above

Skill Sequence
Start with Pale Cascade at lvl 1 and 3
Max Crescent Strike like regular build

Jungleing Path
Wolves,
Blue,
Wraiths,
Golems,
Red

Suggestions
If you dont want to use your Flash or its on cooldown, or unless a lane is overextended i suggest NOT ganking till you have your Lunar Rush. You clear your jungle very quickly and quite effortlessly as well. use your speed and dmg to counter the enemy jungle instead.

Be nice and add only constructive criticism, and please vote! let me know what you think! Thank you.