They can teleport to any place they have already been or line-of-sight.
Tellafragging is not a problem. Rather, if you were to teleport into a solid substance, you're exit point moves to the nearest available empty space.
There is no difference between people how far you can teleport. However, weight limits, speed and recovery time differ between people, rarely getting lower than what you might normally be able to carry or higher than a dozen seconds of hard concentration and a minute of heavy breathing respectively. Outliers always exist, as do the exceedingly gifted.
Everyone in the world can now teleport.
What changes?