At the level cap, or at level-matched PVP, the determining factor is usually the quality of the gear you’ve accumulated; so that e.g. you need to accumulate gear in normal dungeons before attempting heroic dungeons, in heroic dungeons before attempting low-difficulty raids, in low-difficulty raids before attempting high-difficulty raids, etc...
Unless you’re already at the level cap, or doing level-matched PVP, or something.
At the level cap, or at level-matched PVP, the determining factor is usually the quality of the gear you’ve accumulated; so that e.g. you need to accumulate gear in normal dungeons before attempting heroic dungeons, in heroic dungeons before attempting low-difficulty raids, in low-difficulty raids before attempting high-difficulty raids, etc...