They introduce the Ferangi, while they were not suppose to meet up with them until TNG. They introduced the Borg, while they were not suppose to have done so until TNG. They kind of "played it safe" with the Romulans by not letting anyone see their faces or know that they were Romulans or something along those lines, in order not to wreck continuity with TOS where they mention that Romulans had not been heard of for a long time.