Was just thinking today, why more games don't have this feature..?
Im surprised at how much such a little thing can help keep things fresh.
I actually haven't found the game too repetitive yet... And I would say there are certain key elements that have aided this...
1) The amount of options available to the player to customise you character - even if disappointingly it's only cosmetic changes. Would have been nice if ghillie suits affected stealth, and types of backpacks and vests affected amount of ammo you can carry and perhaps movement speed.
2) The Diversity of the landscapes is fantastic. This too keeps things from getting too familiar!
So well done Ubisoft -- and take note for future games...