The use of aluminum will certainly reduce weight by several hundred pounds. We'll hazard a guess that 2015 F-150 weights will range from under 4500 lbs for regular cab models to under 5500 lbs for the biggest SuperCrews, depending on configuration, for the 2015 F-150. The current F-150, for comparison, tops out at 6145 lbs for a Harley-Davidson Edition SuperCrew. Fuel economy should increase as well. Ford is likely aiming past 25 mpg for naturally aspirated models in order to one-up the 2013 Ram 1500.
Of course, unibody construction will reduce capability considerably. Currently, the F-150 carries a payload rating between 2310 and 3120 pounds. Expect about 1500 lbs for 2015 SuperCrew models and 2500 lbs for regular cab ones. Towing will suffer as well, with the minimum likely dropping to 3500 lbs and the maximum dropping to 7500 lbs, if not lower.
As for why the reasoning behind the construction switch, Ford said that "a unibody architecture enables us to better engineer the trucks to suit the demands of today's truck buyers, who expect greater on-road comfort, a quieter cabin, and better fuel economy. Today's buyers are also less concerned with traditional details like payload and tow ratings and are less likely to use their trucks to haul or tow."