Writing my answer without looking at others first. Jasper is an interesting one, because a lot of the main events would have still happened if he'd died, just differently.
-Monty (and Clarke? It would depend... Did he die instantly or did he die back at camp with the 100?) become more hardened.
-No one shoots the Grounders in the tree on Unity Day?? This one is debatable. Would Raven or Bellamy have done the same thing? Even if no one shot the Grounders, it didn't seem like Clarke and Anya were even approaching reaching peace.
-Who would have shot the bomb on the bridge? One of the minor characters? Would they have hit it, or panicked and ran out of bullets too? No one would have brought Monroe or Sterling a gun, if so. Then the Grounders would have invaded and everyone would be SOL.
-Who would have stripped the wire? Did anyone else know how to do it? Could Raven have talked Clarke through it before passing out? Would anyone else have been able to fix it when it didn't immediately go off? If not, surely Finn and Bellamy would have been captured/killed, and the 100 in the dropship could just be starved out.
-Would Monty have gone after Clarke sooner/with Clarke at Mt. Weather without Jasper to tie him down? How would that ripple out? Where would he have hid when Dr. Tsing came in to the room where they were bleeding the Grounders? Would he have died then?
-Would Mya have been targeted with radiation to test The 100's blood anyway, or was she chosen just because she was noticed with Jasper? If they just picked someone arbitrarily, would any of the rest of the 100 taken the chance? Were any of the rest of the 100 getting close enough off screen to any of the mountain men to manipulate them into the same place Jasper was in?
-and most important of all: could Monty have made the moonshine on his own?!?!!