I haven't tried multi-player yet.

On a playability, they did throw in re-playability into the single player experience. You can replay missions in order to maximize upgrades. Within each mission are X number items that you can obtain to help you improve your weapons via black-market arms dealers. Hidden gold, intel reports and books are scattered around the map and the mission area. During your first run-through you are usually so busy accomplish the primary goal that you miss things; hence, the do-again feature.