My interpretation is as follows;
That's more or less it.
Storm Pillar is a bit overrated. It's most powerful effect is that it is a restraining power if placed directly over a foe's head.
For example:
. . .
. E .
. . .
If the effect is placed directly about the enemy E's square, then E cannot move at all (shy of teleport) without taking auto damage of D6+Int. There is no to hit roll.
But, E can just sit there calmly and not move if he has a ranged attack.
But to do this restraint, the Wizard gave up the opportunity to actually damage E or anyone else (on the other hand, certain Brutes and other foes do not have ranged attacks, so it is often autohit or autorestraint against them).
As for sliding foes into the effect, that's a bit problematic as well. 55% of the time, each foe will fall prone in a square outside of the hazardous effect before being slid in.
So, a 45% chance of doing damage (which is typically lower than a Wizard's normal 60% chance of doing damage) plus prone if no damage is done.
But with 4E's design goal (at least in the adventure modules) to have larger areas to fight in combined with diagonal squares requiring a single move, there will be many times where the control is limited because foes can merely move around the effect. But, it's still control.
It matches the Wizard's role of a controller very well. But, it is far from the best first level At Will Wizard power because the ultimate control is by killing the foe (or rendering him unconscious/stunned), not restraining him.
It is, however, the best first level At Will controlling Wizard power and works well with Thunderwave or other forced movement powers.
Also, if an AP is spent, to place two storm pillars, and an enemy moves into the overlapping threatening squares, they take double damage as it is from differing sources. They also get 2 saving throws, and if either succeed, they do not enter the area.
I'm not so sure of this. I don't see anywhere in the PHB or DMG that multiple effects in hindering terrain give multiple saving throws, it's based on the unsafe square, not each effect.
The foe doesn't want to go in that square, so as far as I can tell, he only gets one chance at stopping himself from being moved into it. He doesn't get better at avoiding going into the square, just because there are two effects in the square.
Do you have a rules quote for this?
He does, however, take damage from both effects if he is forced in.