what is wrong whis this?

The List type would be considered a generic but does take a Type that makes up the List, it's strongly typed(unlike ArrayList).

if you were dealing with a List of Transform objects

 public List<Transform> SampleMethod(List<Transform> i)
 {
      return i;
 }

lets say it's a List of strings

 public List<string> SampleMethod(List<string> i)
 {
      return i;
 }

How about getting and returning a list of Transform from a list of GameObjects?

 public List<Transform> SampleMethod(List<GameObject> i)
 {
      i.Select(go => go.transform).ToList(); // Requires using System.Linq.  Returning IEnumerable would make more sense too.
 }