I'm disappointed that they barely nerfed Wall of Fire at all and didn't touch Wall of Ice. Surround someone with the Wall of Ice and just watch the damage rack up; the Wall of Fire can do a crapload of damage if you surround an enemy with it and have you and allies push them back in when they get out.
Wall of Ice didn't do significant damage in 3.5, about 80% of it can be explained as a level lower than Wall of Stone and weak against Fire attacks, so I don't know where you're getting the idea that it racks up damage in 4.0.
In 4E it's higher level than Wall of Fire. It does 2d6+INT damage to anyone who starts their turn adjacent to it. Hitting it does 2d6 damage to you, and each square has 50 HP. The wall does have Fire Vulnerability 25... but that means you have to be able to do 25 fire damage in one round to get through a square in one round. That's not something most opponents have!
So, when you surround someone with a Wall of Ice, they take *at a minimum* 4d6+INT damage. Guaranteed. Meanwhile Blast of Cold (another Wiz L15 Daily) does 6d6+INT on a hit, half on a miss. Sure, it's Blast 5, but you can put the wall so it does 2d6+INT to a few other enemies too.
The thing is, that 4d6+INT is way lower than it's actually gonna do. Realistically, it'll take enemies two, three hits to bust through the 50-HP square... and if they get through on the third round, you just did 12d6+3*INT damage to them, with no attack roll or *anything*.
Wall of Fire also didn't do a lot of damage unless you followed the same kinda idea you posted, and if the whole party is having fun volleying the enemies through a wall of fire once per day then I don't see the problem.
It does, at a *minimum*, 4d6+2*INT. That's at level 9. A single Push 1 power will make that 5d6+2*INT. A push 2 power will make that 7d6+3*INT.
Meanwhile, other powers are doing, what, 2d8, and that's on a successful hit?