I don't know if the OP should be thrown about her, BUT I really do find it a problem that her base scaling is SO high. Without any items she outclasses a lot of assassins in things such as health, damage and general tankyness. This I feel is the real problem with Diana. I mean when you look at her stats on the Riot provided page you can quickly notice that without items she is tankier than both Morgana [86.6 hp/lvl] and other incredibly dense Mage casters, but she is claimed to be an "assassin" (Even though Riot doesn't tag her as such most of us have considered her as such considering how powerful her ganking powers are.) and when we look at assassins like Kassidin he is incredibly squishy [52.3 hp/lvl] vs. her [90 hp/lvl]. When her stats are laid out, she really just has a lot of everything. I'm not really throwing out OP here, but it feels like Diana can do well in too much without building anything really. Gaining natural MR, Defense, Health, Damage all at decent ratios per level, and to those who say build MR please remember Diana can go in any lane, so early counter building requires you know where she is going first hand, and considering that her Q at Level 1 hit me for aprox. 180 when she merely had a doran's ring I feel like you need to have that MR as soon as you lane against her. (If I recall correctly I was using MR per level glyphs so I had wee bit at the time, as you one would expect.)
Please Riot, not asking for a massive nerf here, maybe just lower her scaling a bit, force her to build a crystal scepter or something if she wants to have quite this much health.
Diana's Stats below for the curious who don't want to search for it.
Damage 48 (+3 / per level)
Health 438 (+90 / per level)
Mana 230 (+40 / per level)
Move Speed 320
Armor 16 (+3.6 / per level)
Spell Block 30 (+1.25 / per level)
Health Regen 9 (+0.85 / per level)
Mana Regen 7 (+0.6 / per level)