IMO, I think Glacial Storm should:
--cost more MP/sec (150%)
--have a smaller radius (30-50 less)
--have lower AP scaling (15% instead of 25%)
--do more base damage (200%)
--slow more/linger (2 sec)
--get a little more range. (+100)
As far as an ult is concerned, it's pretty underwhelming in flashiness and early game damage. Honestly, Anivia is already great at initiating with a Flash Frost stun, since a double hit does a lot of damage, and chills your opponent. Giving her an initate that is AOE, does a lot of damage, and drains her mana pool extremely quickly means that she fits into the glass cannon role even more. I think managing her mana pool leads to powerful single-target pressure, and GS gives her a mana dump that is good AOE fight utility.
Her low MS matters now matters less; if she's initiated with GS, she has all the MS she needs to catch whoever is in the storm. But the longer she keeps it on, the less likely she'll have the mana to finish the job when she's close enough to hit. Turn it off, they still have a chance to get away, but you have the mana to finish it.