How different is it each time? if you are able to quantify it (not necessarily with numbers)
It can be very different.
When you're creating your character you choose a "background" for him. The background is the lifestyle or profession your character had before the beginning of the story, so it influences your starting reputation with the different factions.
Each background also has it's own unique start. For example, if you choose Assassin, you're sent on a mission to murder someone, but if you choose Merchant, you're sent to negotiate a contract. You can switch to a different faction or profession later though, so you could make a big, dumb merchant (not very good at being a merchant, in other words), and then join the Imperial Legion as a soldier.
It's impossible to see everything in any one playthrough. There are entire areas that will become available for certain backgrounds, but not for others. It's also possible to get different outcomes from failed skill checks, so just being good at your skills won't let you see everything.