They've basically re-written the vast majority of the infinity engine, squashing age old bugs as they go. They've firmed it up, planted a fresh UI on top, made the multiplayer more robust, added content, added dialogue, made some tweaks (checkout the revised weapon proficiency chart versus how it was in BG2) and included a load of mods.

All in all it's the original... enhanced.

If I remember right, the developer has said that initially it will be like it used to be - you connect directly with your mates (LAN or Internet). They plan to throw up some servers and do some sort of official way to for multiplayer groups after release though.

As for how it works, one player will have the main PC and they will do all the plot dialogue, etc. Things like pausing, talking, etc are set as permissions by whoever is running the MP session (whoever has the main PC). Might be that your buddy leading the party wants to do all the talking so he can deny you access to that. Otherwise you basically do what you want as long as you keep near your party.

I'm no expert though, having never done a proper multiplayer playthrough before!