Subvisions and Poly count. For animation you can crank up the poly count to get super smooth looking meshes, which will increase render time, but give you a really polished finish and give you more "play" for tweaking the mesh for extreme bends, like at the arms and legs. Also more dynamic facial animations. In game engine though it's all about conserving poly count. You want to achieve the full range of motion needed with the lowest poly count possible. This will affect how much you can fit in a scene, how fast the game loads and how smooth it runs.