Well,
I think it can best be described as D20 Modern-like. Playing it would feel a lot like D20 Modern but there are some specific rules that are different. First hit points are a lot less. Generally you only get a point or two per level rather than a die roll. I also don't think you get your CON bonus each level either. Autofire works as a AOE attack against 10' radius. Characters start with the Mobile Infantryman class and can then go into advanced classes later like Sniper, Comms, Field Medic.
There's some weirdness with the classes. The MI class is 10 levels and there is a 10 level Veteren class that you can enter only if you take all 10 MI levels and none of the other advanced classes. However most of the advanced classes are 3-5 levels and this includes the Officer Training Corp. So if you specialize you end up forced to take a bunch of different specialties if the campaign goes up to 20th level.
Admittedly this isn't likely to happen in a SST game but I did notice it.
In regards to Mongoose's poor history with editting I've only found a couple mistakes. Amatuer spelling mistakes admittedly but still for Mongoose that's pretty good.
Jack