From their website:
Right off the bat, Heroclix fans will notice are several new sections have been added to the Players’ Guide. This change helps highlight the inclusion of Alternate Team Abilities (ATAs) in the PG as well as acknowledge their greater role in the overall game. Throughout this list of updates, we’ll pepper in some of the various new cards. Please note that while the Marvel ATA downloads are not posted, the information you need to know to play them is contained in the Player’s Guide.
Players will also notice that some items have been removed from the Players Guide. This is due to the inclusion of those elements in the new 2010 Heroclix Core Rulebook. This is all part of our ongoing mission to make the Players Guide as sleek and sexy as possible, as well as easier to use.