Her auto attack feels very, very, very bland and terrible.
Her passive is, it's nice, but it should either stack or increase attack speed scaling every 2 or 3 levels.
Her Q is fine were it's at except for it's ratio, nerf the ratio either down to a flat 7, or a 6.5 or increase it's missile by a tiny bit.
Her W costs WAYYY to much mana, BUT it's balanced by the fact that it heals and deals damage (high base which is good), and a shorter CD than Soraka's heal, but again it costs alot of mana so it is balanced.
edit ; just read the OP, thank god.
Her E (Or passive) should increase attack speed. Not by a whole lot, but a bit like 2%/4%/6%/8%/10%, the whole extra magic damage is nice, but it doesn't do a whole lot compared to Lulu's passive.
The slow on her E compensates for the nerf on phage that corki and ezreal build, so i'm sure those two will be her BEST lane partners. Not to mention MF and Draven.
Oh god Draven + Nami = GG.
Her R, is fine where it's at. The fact that it goes across the ENTIRE lane makes up for how slow it is. Only suggestion is to increase knock up depending on how far the tide traveled.
Her mana pool
Nami 348(+43/lvl) 1122 @ lv 18
Lulu 200 (+50/lvl) 1100 @ lv18
Janna 302 (+64/lvl) 1454 @ lv18
Soraka 240 (+60/lvl) 1320 @ lv18
Sona 265 (+45/lvl) 1021 @ lv18
As you can see the "spammy" support champions (Lulu/Sona) get lower mana pools with lower CDs and high mana costs, and the other champions get high mana costs with low CDs but with higher mana pool.
Consider buffering her mana pool.
Also, she's not that great mid. She is defiantly 1000000 times better in the support role, and her team fight presence is immense.