Why is it so overpowered at the moment? Its a once an encounter trick, that another class could use completely every turn during the same encounter. With the way the At-Will powers of any classes are, including "features" which are listed and described as powers in their own text, such as Lay on Hands, it a maybe useful skill that will help in a pinch.
Halflings, Eladrins, and Elves all have a per-Encounter ability which are all useful on top of other racial traits that make them desirable. Half-elves only have Dilettante to their name with an option to get features from Elf and Human requirements, which aren't too exciting, imo.
The worst offenders that I can see being worthy of watching are a Half-elf non-Warlock taking Eyebite or a non-Paladin taking Lay on Hands. That is one turn of invisibility against one monster till the start of your next turn, if your attack lands. Or spending a Healing Surge and healing yourself or another once per encounter as a minor action, for either 1 to Wis Modifier times a day.
Eyebite would give a Ranger or a Rogue a quick chance to run off behind cover where one opponent wouldn't notice, or grant a Rogue by himself Combat Advantage when he uses an Action point and spends it using Eyebite followed by an attack.
Lay on Hands healing sounds powerful, but limited as well. Using a Healing Surge as a minor action per-Encounter is something limited to...Dwarves, Warlords, Clerics, and Paladins(At-Will for them). Unlike a Dwarf though, they still have their Second Wind to use as a Standard Action. As its limited to a number of uses per day, from 1 or Wis Mod, which ever is higher, the classes which don't get more then 14 Wis would basically trade in the encounter for a daily Healing Surge use. The high Wisdom classes I can think of off the top of my head would be, Clerics, Rangers, Combat Superiority Fighters, and Wizards. Other classes would loose out on bonuses to their own abilities to get one more use out of Lay on Hands.
And for those At-Will Rogue utilities which also fall under as a choice, a Fighter getting Great Leap or Nimble Climb once per Encounter is overpowered just how compared to other races normal features?
So, what At-Will ability turned Per-Encounter would break a class or the game?
Well, maybe a Rogue using Mage Hand for Thievery Skills.
In my mind, the feature powers of classes were setup the way they were so they would not get caught behind the 2 At-Will limitations at the first level or taking up one of the encounter or daily powers. I can still see them counting as level 1 abilities as they are recieved then.