Originally Posted by Aikimiller
As a rule of thumb, a split pusher needs
1- to be able to push fast and hard
2- a quick escape mechanism, and good mobility
3- to be either non-pivitol in a team fight, or able to get back to the team fast enough that you can't tell he's gone (Shen, TF, Pantheon)
Also an important ability is to be a good dueler.
If the enemy team is forced to send two people to stop you because just one would get killed, then it's great for your team.
If the enemy team can stop your split push and kill you with just one champion, it's usually pretty stupid to be trying to split push in the first place. There are exceptions, of course. Like if that one champion that can stop you is a critical part of the team fights, and your 4 teammates can steamroll his 4 teammates in the absence of you two.
But in general, better duelists are better split pushers. For example, take Karthus. He can push really well and can impact a teamfight from anywhere on the map with his ult. But he's (usually) squishy as heck and is a lousy at 1v1, so if he split pushes at the wrong times, it's very easy for him to get caught and killed by many champions. Ones that can hop over his wall, at least. It's pretty easy for him to escape from champions without any dashes because of the retarded slow on the Wall of Pain.