Hasbro wants the X-COM games to be part of a long and continuous history. We have action sure, but strategy, adventure and RPG elements too. Alliance's gameplay pattern is fairly unique and we're proud of that fact. In changing the game model, you risk rubbing X-COM fans the wrong way. That's about the bulk of the major changes.
Alliance is also a puzzle game.
Thus, our puzzle system. A lot of it depends on the troopers a player has selected for a mission. Some troopers can become more versatile than others, and some puzzles can be solved by multiple skillsets. There are rescue missions, escort missions, intelligence gathering missions, and straight up combat missions, to name a few. In addition, each mission contains its own set of tactical problems to overcome, not all of which are disclosed to the player in the mission's briefing.