The amount of time between the eras of the original Star Trek and The Next Generation would, on the face of it, suggest that fans of the former series would be disappointed if they hoped to see characters from the former show up in the latter. Of course, in the worlds of science fiction, everything is possible, leading to these five crossovers between the old and the then-new designed to celebrate the franchise's history.
Encounter At Farpoint
The idea of launching a new Trek without some kind of nod to the original seemed unthinkable at the time, and there was something about the low-key cameo from DeForrest Kelley as an aged and even-crankier-than-usual Dr. McCoy that seemed particularly fitting. He brought a wonderfully funny cynicism to the overly sincere, optimistic proceedings that may just have reflected some of the audience's feelings as they watched the pilot for the first time.
It took two seasons for the show to return to source material from the original (In part, perhaps, from backlash against "The Naked Now," which was essentially little more than a rewrite of the original series' "The Naked Time"), and it did so with a character who could be described as little more than a minor character from the original: Spock's father, Sarek. The result was something surprisingly moving that didn't rely on any "OMG, It's That Guy From The Old Show!" revelation that nonetheless connected the two series together slightly further without stepping on the toes of the still-ongoing movie series at the time.
Unification (Parts 1 & 2)
With the 25th anniversary of the entire Trek franchise upon them, the writers and producers of TNG knew that they had to come up with the goods for the show's fifth season, and a two-parter reintroducing one of the original show's main characters seemed like a great idea at the time. Sadly, the finished product gets too bogged down with political infighting and a story that seems too talky to be entirely successful, making the whole thing seem overlong and a slight letdown, really.
A season later, and things got better when the original show's magical engineer managed to bridge the decades between series with the aid of a jury-rigged transporter. Like "Sarek," "Relics" plays things more low-key and more melancholy - Not entirely without action and intrigue, because this was a Star Trek show, after all - and is all the better for it. Maybe my favorite of all of the original Trek crossovers with TNG, this is also one of the better episodes of the show's final two seasons, and a pretty great example of Trek keeping things turned down in general.
Star Trek: Generations
And then, there's this: The first of the TNG movies and… maybe the worst? Actually, considering Nemesis, that may not be true, but Generations is the kind of movie that can best be described, politely, as "uneven." Perhaps it was exhaustion brought on from having to write this so close to the (in almost every way superior) "All Good Things" finale to the series, maybe it was the numerous rewrites and story changes that happened along the ways, or it might even have been that the presence of William Shatner just managed to undo some delicate balance, but the final adventure of James T. Kirk entirely overwhelmed everyone and everything else in this outing, and made it just plain a chore to watch. Everyone involved deserved better, especially Malcolm McDowell, who could've been an awesome bad guy if given the chance...