I mentioned this in another thread, but I'll put it here as well.
Helper functions/function overloads for when a transformation uses angles in degrees that would accept a radian argument instead and do the conversion would be a nice feature.
I know it isn't difficult to write the helper functions separately, but it's an extra thing to remember, along with doing an extra function call or multiplication every time they are used. An overload (possibly with the postfix character R, so a Rotate(float degrees) method would also have a RotateR(float radians) variant) to do this with an inline class method would make the conversion simpler to remember, and make SFML more appealing as it doesn't enforce a particular convention upon library users regarding how they should use angles