I would have to protest the changes to the Archangel Staff's insight ability. The additional AP provided by the bonus of insight is not currently a unique passive, and provides magic users with the ability to sacrifice HP and other stats with the opportunity for a little extra damage. In addition, some spellcasters need the extra mana offered by holding more than one Archangel's staff. Even with the extra 1000 mana, and with 4x Archangel Staves at level 18, I find myself often running out of mana when I am without a blue buff.
There is an obvious downside to going with a 4 archangel staff build - low HP and lower survivability, the lack of the slow from Rylai's, but it does provide a playstyle that would otherwise not be possible, and it provides AP carries a build that allows them the possibility to take down fully built ADCs. There is still the rock paper scissors effect, a character with high AP cannot burst down a well built tank (indeed, if you have 4 archangels, any tank that gets ahold of you is probably going to throw you around like a little ragdoll. But hey, some people are into that too, it's not my place to judge.)
Keep the balance, keep the possibility open. It would make me very sad to finally go pro and not be able to display my skills with my build.
PS, I think Manamune should also be adjusted to NOT have the Mana -> AD conversion be passive.
Update: You can still stack 4x AA's to Seraph's Embrace to get a bonus 4,000 mana, but the difference in total AP between the old and new stats would end up being 249.6 AP less with the new build.. This converts from the current AA Mana of: (4000*0.03 - (400*4+1000)*0.03*4)*1.3. That's (new - old)*Adjusted for Deathcap. I can understand why the passive is made to be unique in light of the 4000 from the Seraph, but I feel that there should be some modification done to keep the new items power in line with the old items, as the new items can certainly be a bit of a nerf to those whom are able to position themselves. And for those that complain about Anivia being OP - do you honestly want to give her more survivability by giving her a mana shield? I would also wonder about the gold costs, since the AA is so costly already. However, I would propose a solution to this - apply not only the option of a seraph staff, but another upgrade path that still keeps the option of the heavy AP all balls in playstyle alive.