To start the 'story', you've got to do one or more special mission for a lord from Poland, Moscow or one of the Cossacks, maybe you just have to get rep up with one of these lords not sure..At any rate, got my rep up to 30 with one from Poland and he offered other quest and convo lines that began the story.
Concerning the story, unlike Warband, the story is about these three factions mostly, the other 2 are sort of bit players, and not about the player. I think thats is why the stock game pigeon holes the player, and stops you from marriage and from starting your own empire..though god knows why kept the stupid rebel faction leaders that you can try to put into power, unless they are hardcoded into the game engine...which may well be the cases lol, considering most mods I have seen make these characters invisible rather than remove them.
On mercs, if you have room for them in you party (Jackrabbit), they are added instantly. Also, upgrade their weapons ect at the merc camp, otherwise they are just about worthless
On tactics, I've been keeping 10-20 inf, marks, and cav. Send inf in to screen marks, then lead a cav charge yourself after the ground units are mixing it up, so that your less likely to get blown away
I tried a siege, seems like its basicly the same as any other mod'd M&B, my inf got hung up on jagged graphics, my low level marks mercs killed 2x their number of enemy elite cav and melee, but we lost because I was shot while trying to take part in the action myself, so as is, F&S requires you to play like a commander not the MVP
Edit: Units do not level up, which is very lame IMO
All said, this one is getting shelved until mods come out