They have plenty to differentiate themselves with, the problem is the
second hand market of Canon and Nikon lenses and pre-existing userbase.
SLT cameras (and therefore Phase Detection AF in video)
Twilight mode and other similar features which are fun, if not amazingly useful
SSS
a-mount lenses
Best "value" full frame body out right now (a850)
Burst rate (even ignoring the SLT's)
I don't particularly care whether the lenses are cheaper are not - I'd happily pay an extra £50 (even that doesn't look to be the case given Sony's current pricing) to add SSM to every single one of my lenses that I ever used.
I can't imagine Sigma would be particularly willing to, but I'd say a Sigma/Sony partnership in making a-mount 3rd party lenses more available, and reducing Sigma's need to reverse-engineer the mount data, could help with the lens market problems, though the
second hand market would remain poor for a while.