Couple questions.

A: She got nerfed a while ago, and DISAPPEARED. Like, instantly. What happened? Did they make her worthless, or at least sub-par to everyone else?

B: In what situations would Diana be a good pick? Like, who does she counter, who does she work well with, etc.

C: How would I go about building her?

D: What's her playstyle? I imagine she plays similar to Akali.

I play her as a jungle ans that's the only thing I've seen her do. If your going to mid with her I would guess she would do ok as long as viegar isnt there lol

A: http://www.leaguereplays.com/replays/match/2098018/ <--- no
B: http://www.championselect.net/champ/Diana <--- strong/weak against
C: AP/Health (or resistances)
D: Aggressive seems to work well

Im pretty sure she just matched up against a Jayce solo top lane in the world championships and held her own, which isnt an easy thing to do against a good Jayce.

I've still used her mid and she wrecks consistently. Many champs scare the **** out of me late game (bad Eve's are terrifying once they have a DFG) but she is a better melee mid assassin than akali because of her Q. I have taken on akali, brand, eve, ahri, and a bad LB (not sure how a good one holds up to her). Gragas and Morg gave me the most problems, but using my ult as a dodge mechanic for their skill shots was effective; their tankiness saved them in a lot of cases unfortunately. But I have no issue with her, even when my team loses I at least go even.

EDIT: I also stack MR early, which is really helpful, starting with merc treads, two dorans rings, and then abyssal, it kind of ruins their day