Christina Norman, BioWare's lead combat designer, breaks down the retooled engineer class in Mass Effect 2 in an IGN interview.
Engineer players who hack into and control mechanical enemies will find their control over their pets is more complete - more like a "charm" ability seen in other RPGs. Engineers can also summon combat drones and, added to hacked enemy AIs, plus two squadmates, the engineer will command the largest force of any class.
I think Engineers will really appeal to players who like thinking their way through combat. More than any other class, the Engineer has the ability to shape the flow of battle by summoning, hacking, and debuffing enemies. For a master engineer, shooting an enemy is seen as an overly simplistic brute force approach to battle.
There's much more revealed in her discussion, plus gameplay vids, through the link on IGN.