Before we go much further, let me just mention the title of this episode. It's another call and response from the original series, but one that like the episode itself is perhaps the first sign since Q that The Next Generation is going to be a notable enterprise all its own. It comes from the famous mission statement that Kirk and Picard speak over the credits of every episode, with that one crucial difference, "one" instead of "man," just as years later Enterprise would eliminate the split infinitive concerning the boldness of the adventures when essentially having Zefran Cochrane creating it. As with the later example, it's Star Trek trying to grow up a little, just as in the casting of the older Patrick Stewart to be the captain of this new crew. Of course, there will always be fans who suggest such quibbles aren't necessary to fix.
Anyway, so "Where No One Has Gone Before" is the best Wesley episode of the season (arguably the only good one). It's the introduction of his cosmic destiny, something that wouldn't be visited again until the final season. Had the series been launched later, it's very probable that subsequent creators would not have allowed such an important character development to be overlooked. It's arguable that this is a lot of what the series should have been about, and that Deep Space Nine, especially with Sisko's relationship to the Prophets, that Star Trek itself later figured this out.
The Traveler, then, is a lot like Q, Next Generation introducing a giant concept that could very well have been like something out of the original series but instead transformed into an integral element of the mythology that shaped later franchise developments. On the surface he's just the catalyst for crazy warp engine experiments (tagging along very much with one of those original series misguided-Starfleet-officers-of-the-week) that push Picard's ship into one of several far-out journeys into unknown territory (happens again in "Q Who?" and eventually becomes the premise of Voyager). But when we find out that it's the Traveler making everything happen (unfortunately while being exploited, which is another nice aspect of the episode), and that he's marked the erstwhile annoying boy genius Wesley as another like him.
Q was the first to suggest that Next Generation was about taking Star Trek to the next level, figuring out how to not only explore strange new worlds but to, well, go where no one has gone before. Later seasons, although technically superior in every way, lost this thread and later series never really got it back. That's one reason why this episode still stands out.
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Memory Alpha summary.