I've played for 18 hours and have only completed 18%.
Really? Have you done any missions so far? This doesn't seem right. I had 7% done after completing one mission and an assassination mission, which are really pointless by the way. For a mere 6 or so hours of screwing around and doing some missions I was at 35-ish %.
This game definitively is short. Near-end upgrades completely kills the challenge in the game, I suppose it's their way of letting you know you've finished the game.
My character haven't finished the "story" missions (but close) and it can't die from anything except vehicle and melee damage.
If they balanced out the upgrades and made it a longer game I'm sure it would have been a much better game. The vehicle balance also comes to mind, the steering of SUV's and large vehicles is so unnatural and hard to steer.