In FS2, you press a key and you will be propelled to max velocity until you either choose to decelerate or hit something. As far as I know, there are no strafing controls and no way to go backwards without turning around first. Whereas Assault has a more two dimensional axis where enemies typically come from left or right and you are autocorrected to face the center when you release the stick, FS2 will let you move in any direction you choose relative to the starting angle. If you wish to go straight up or straight down, the FS2 engine will let you do that. It also means that enemies can do this, too, allowing them to come from any direction in SoL.
With further thought, though, if you fully stop and hit the afterburner key, you have to hold it or you will stop again immediately. You could use either scheme, depending on your preferences in most situations.
Back on topic:
The videos that I have seen look very smooth at reasonably high resolutions. What kind of hardware are you running SoL on now?
Will there be a separate installer for each platform, a universal installer, or a drag-and-drop install compatible with FSO through a set of VPs? (Already partially covered)
How complete is the project? As a percentage?
Will there be an addon system for user-made campaigns? Through FRED2? For Mac users left out in the cold without access to FRED?
How difficult will SoL be compared to the main FS2 campaign at the same difficulty level? (Disarming the Sathanas comes readily to mind...)
Will we be able to customize our weapons load or will it be locked to defaults?
Will we be able to pick up bombs, guns and shields/armor floating in space?
Will the Energy Transfer System be utilized at all? It's a massive help in FS.
Will there be ground missions, or just space/air missions?
Some campaigns drastically change the speed at which ships move, such as BtRL which changed the average top speed from about 75 to 300, while a few FS1 campaigns poke along at 50-60. What is the pace and focus of SoL? On soaking up damage or dodging it? On outflying, outgunning or outthinking your foes?