There are big changes coming to HeroClix Rules, and they all take effect May 1st 2011. Over on the official WizkidsGames.com you can read new articles describing those changes in detail. Here at HeroClix World we've create this summary of changes spoilers section, which is basically a summary of those articles. This will make it easier to read it all in one places. We'll keep it updated as needed.

CANCELING (GENERAL)
Currently you can cancel your own power or team ability on your turn, and it returns at then end of your turn. This is particularly handy if you have a character with Willpower and want to cancel it in order to take push damage (example, Monica Rambeau can cancel her Willpower in order to push to a better click!)

The NEW rules will remove CANCELING altogether. You simply can't do it. 

CANCELING (STEALTH)
The ability to CANCEL (see above) was useful for Stealth characters, because you need to CANCEL your own Stealth in order for your allies to use Probability Control or Perplex on it. With CANCEL being removed, they will have to reword this ability. From WizkidsGames.com: "Stealth is now a power that only prevents hindered lines of fire on your opponent’s turns, not on your own turn"

I suspect they'll also need to modify Charge (which allows you to ignore knockback unless you cancel it), and a few other powers.

FREE ACTIONS
No need to say "Once per turn" anymore for free actions. If your character can use Outwit and Perplex, both of which are Free Actions, then that character can use each on your turn, once per turn. This will make the current Powers and Abilities Card a little smaller, and will help clarify feats like NANOBOTS -- currently you can use that feat multiple times per turn since it is a free action; in the NEW ruling all free actions can be done only once per turn... (unless otherwise specified).

From WizkidsGames.com: "A character may not activate the same game effect twice in one turn with free actions, unless the game effect indicates otherwise...  If a character had 2 game effects that each allowed them to use the same power or ability, they would still be able to use it more than once. "

DOUBLE POWER ACTIONS
This is a pretty interesting change. Just remember that a Power Action is still a Power Action. So if something prevents you from using a Power Action, then it also prevents you from using a Double Power Action.

From WizkidsGames.com: "To use a double power action, your character would need to be cleared (no tokens assigned). As you resolve a double power action, instead of giving the character one action token, you’d give it two, instead. These tokens would deal pushing damage normally....

Example: "Super Splash: Give Gidget a double power action when she occupies water terrain. She can use Quake as a free action and deals penetrating damage."

WILD CARDS / MULTIPLE FREE ACTION
New rules specify that Wild-Card-Usage is a Free Action. However, Wild Cards is a special ability that allows multiple Free Actions per turn. So you can still use your Wild Card multiple times in your turn. HOWEVER, you can no longer use it during your opponent's turn!