There is one major event that happens between Enterprise and The Original Series that has not been directly depicted onscreen: The Earth-Romulan War. What little official canon we know about the nature of this war comes from Spock's remarks during TOS:
As you recall from your histories, this conflict was fought, by our standards today, with primitive atomic weapons and in primitive space vessels which allowed no quarter, no captives. Nor was there even ship-to-ship, visual communication; therefore, no human, Romulan, or ally has ever seen the other. Earth believes the Romulans to be warlike, cruel, treacherous... and only the Romulans know what they think of Earth.
Photonic torpedoes (not photon torpedoes) were introduced at the end of Enterprise Season 2, so initially it would appear that this description is pretty far off. However, Memory Alpha does note that the type explosive used in spatial torpedoes has not been specified, and they were seen in the Enterprise's armory in the final season when conflicts with the Romulans were beginning. If they were classified as atomic, then Spock's comment could be seen as a simplified history. The same might be said of the rest of it, as well.
The lack of ship-to-ship visual communication seems to be a direct contradiction to what was shown on Enterprise, given that it was standard throughout the series, unless something happened that caused Starfleet to temporarily give up on such communication channels.
While non-canon, the novels did introduce some Romulan technology that could explain this.
Near the end of Enterprise, in the three-parters 4x12 through 4x14 (Babel One, United, and The Aenar), we learn the Romulans are experimenting with telepresence-controlled ships, using Aenar as their test subjects.
The summary of the ENT novel Kobayashi Maru, which takes place after the end of the TV series, indicates the Romulans have a new technology based on telepresence, called telecapture. They'd successfully used it to hack in and take over Klingon and Vulcan ships, and planned to use it on the Enterprise as well.
Two simple ways to defend against such technology immediately come to mind:
- Eliminate access points (such as reducing external communications to only what was necessary)
- Replace electronic components with mechanical ones, so they can't be modified without physical access (as seen in the NCC-1701's design)
So Spock's description of the Earth-Romulan war may have been literally correct, not because of a lack of technology, but rather as a defence against a new weapon. Post-war, when little was still known about the Romulans, the mechanical controls seen in TOS could have been maintained to avoid risking the telecapture technology being used again.