As a prequel to the Trek franchise, Enterprise was jam-packed with continuity porn. Some of it was so subtle I missed it until I read about it online (alien races met in the first season were namedropped but never seen in TOS, such as the Axanar), others attacked the viewer with the subtlety of a sledgehammer wrapped in barbed wire ("Maybe.... artificial life forms. It may take a few... Generations"). Which are your favourites? -Arik Soong's "I call it the 'Briar Patch'" -Zefram Cochrane originating the "Captain's credo" in "Broken Bow" -Andorian ships having the TOS-style double front console and Vulcan ships having classic movie-style console graphics. -The Augment arc, an excellent Khan prequel (and with unintentional yet awesome foreshadowings to the latest movie, such as the mention of Archer's father, and effortless Klingon whuppings) -The Klingon forehead thing. Totally unnecessary, but totally awesome. When the female says she felt fear for the first time since she was a child, it also explains why TOS Klingons acted so much like villainous humans and so unlike the movie/TNG/etc versions.
In "United", T'Pol says "One man can summon the future", which might "foreshadow" Mirror Spock's "One man cannot summon the future", which we first heard over 30 years earlier.
Not my favourite, but that the Malurians in "Civilisation" are a subtle reference. Sometime after, I spotted that as one of the systems eventually wiped out by "The Changeling" Nomad probe. The Coridians from "Shadows of P'Jem", whose admission into the Federation was being debated in "Journey to Babel". Although they dropped the ball with their completely different, more alien-looking Ambassador seen in "Demons".
During first run, that was the first episode I actually liked, because it felt like a TOS story. I caught the Malurian reference as soon as it came out of the guy's mouth.
One of the episodes had a scene where Hoshi walked into sickbay, to find a tribble. She finds it cute, just before Phlox feeds it to a creature in a cage. I can't remember which one though. Something from Season Two perhaps?
Even better than that, we find out that the Briar Patch and Klach D'Kel Brakt (where Kor once famously battled, and re-enacted in one of Quarks' holosuites) are actually one and the same.
It also fits Worf's reluctance to discuss the differences in 'Trials and Tribulations'. For such an honor bound society to mutilate itself trying to steal abilities from another (inferior) race's experiments and be reminded of this every time they would even look at each other would not be one of the high points in Klingon history.
Indirect foreshadowing, which is that the romulan and vulcans are far closer in behavior than is shown in tng.