FreeplayStart completely removes the main storyline and barriers from the game. When you start a New Game you are taken to what is basicly a character creation screen. You choose a name and then which faction you want to join (Mercs, Military, Loners etc).
When you start the game, all of that faction will be friendly, their enemies will be hostile etc and you start in that factions base (All Factions have a Trader now too). The mod also has a 'Disguise' system meaning you may be hostile to DUTY but slap on one of their Uniforms and they'll be friendly to you till you take it off.
It also has advanced...pathing...stuff, meaning the AI can end up miles away from their base. I followed a Milifag patrol from the Cordon base to Red Forest and only 1 got killed.