According to TJ Wagner of Microsoft, "in MechWarrior 4 you are in charge and will be making the decisions", and "for the first time, the player will experience the BattleTech universe as intended by the creators".
"The player will be able to feel the sheer size and weight of piloting a 40ft tall, 80 ton war machine. The world has been populated with highly detailed building, vehicles, culturals, and fauna to constantly reinforced the size and power that piloting a 'Mech brings. Weapons and environmental effects will bring the world and war to life for you."
To find out more, read the interview with Wagner over on 3D Action Planet, which also (briefly) covers the game's plot, configuring your own mech, and more. It also brings confirmation that the new game's multiplayer mode will be client-server (like a first person shooter) rather than peer-to-peer (like Mechwarrior 3). Good news indeed.