Its a bit of a different thing between the Elves and Humans (by design, obviously). The elves have a relative bounty of spellcasters, so when the Eldermage comes out, he can create an obscene amount of mana, which then in turn is used by the other spellcasters, who then in turn help generate more mana. Its a neat trick when in works, but its really not that hard to set up. One Eldermage in the backfield and two spellcasters within three leagues, and you are well on your way.Place a seconf Eldermage in the vacinity and it gets wonky.
On the other end are the Humans, who dont have an easy way to generate mana besides mana pools and valour. This makes mana a much more precious resource for the humans.
Honestly, a Stormfriar equivalent in an Elf deck probably works with 30+ mana, simply because its not that hard for Elves to get mana.
A simple nerf for the Eldermage is to remove the distance aspect to his power, make it only those in his stack. It makes it still super useful, but does take away its uber-utility quite a bit.