Glass windows

The biggest weakness of glass is that they don't make very good
radiation shields. O'neill's design called for mirrors shining
through glass, but in our everyday lives, we wouldn't need "real"
sunlight. In fact, someday there will be interstellar colonies for
whom there can be no "real" light, only "artificial" light. Windows
will be necessary in the early days for aesthetic reasons, and there
will always need to be viewing platforms, scenic overlooks, etc. But
artificial light can do most of what we need inside the colony.