A significant change to game mechanics that is also being introduced that greatly affects combat, power use and actions is the concept of a personal energy pool, I call this your characters mana pool. The introduction of this is a complexity, but the pools are fairly low numbers and honestly its no more complex than tracking which powers have been used, and its really not much different than the psionic Power Points already being used by some classes.
The Mana Pool idea is commonly used in many games as a means of tracking a characters inner power reserves, some games call this stamina or fatigue, but Mana is the term I prefer. Unifying all characters under the same tracking mechanism was a must for a universal system, and it has other advantages too. Now, characters can use the same encounter powers multiple times, they only need to have the Mana to do so, same holds true for Daily powers. They simply cost more to use.
Also, having a single source of tracking character reserves also allows more general abilities to be given a Mana point value as well, so gaining an extra minor action in combat simply costs Mana, as does using a Move action for a Minor action slot.
This allows greater flexibily in combat, where a character could take a Move/Move/Standard by converting their minor action to a Move for 2 Mana, or could take a Minor/Standard/Stanard by converting their Move action to a Standard for 3 Mana, or even perform a Minor/Move/Standard/Minor be converting a Minor to a Free Action for 1 Mana. This added greater flexibility than the Action Point system, and players can be more creative with their combinations.
GM's can put rules in place to govern the use of how many Mana may be spent in a round. These should only be put in place when the group is seeing imbalance in the Mana pool uses by a Character charging into an encounter and continually dumping all their Mana on their first action. Personally in play-testing I didn't see this behavior. Players typically would use their Mana conservatively and most often finish the battle with Mana to spare, or purposefully held in reserve for a get away or major power to be used.
Plus, this adds a completely new element to game mechanics where before everything had classified as an At-Will, Encounter, or Daily, or there were the occassional effects that had the added cost of a Healing Surge. Now balance between various effects can be tweeked, if a typical Encounter Power costs 2 Mana, and a Daily power costs 6 Mana, then powers that are seen as "more powerful" or "less powerful" can be adjusted. A more powerful Encounter Power may cost 3 Mana, or a less powerful Daily could cost 4 Mana.
On top of this Mana Recovery becomes key, items such as Mana Rings could be warn to supplement a characters reserves for those particularily demanding battles, or Mana Potions like those found in our favorite computer games. Rather then an Action Token for a milestone, a bonus 5 Mana could be given (the cost of a additional Standard Action), to be spent by the player as they see fit.
In addition to these items to allow Mana to be recovered faster there could be "Dampening Zone" that reduce Mana recovery to half or a quarter its regular rate, or complete "Dead Zones" that prevent any recovery at all, or the "Energy Zones" that boost the rate of recovery. Rather than entire area of the realm with these zones the adventure could crafted such that it's largely dampened recovery, but one room has an boosting effect, or individual items, statues or fountains that give a one time boost. These could all be used as a scaling effect for a GM to set the pace of a dungeon crawl. Longer crawls may have boosting effects/items around to allow the party to go more without needing to rest, where shorter adventures can have dampening or Mana draining effects that make each Mana spent/lost critical.
I'm working on this chapter of the Core Rules now, and it seems to be coming to gather nicely. A few more rounds of play-testing are required to finalize it.