There are plenty of things that make Trek great — otherwise it would hardly be such a huge success, lasting almost forty years and seven hundred hours of drama. But as many people before me have argued, the one quality above all others that makes Star Trek great drama is the fact that it's more than just simple entertainment... it's about something — it's about us, Humanity. Its introspective. Looking back on the most recent years, I think that Trek has not always kept straight and true to this path. This might explain, at least in part, why support for the series has been gradually washing away these past few years.
The original series' strength was in its optimistic, hopeful outlook. The writers managed to convey the basic hopes for a better future of a united Human race in almost every single episode to some small extent. Although in the early 21st century it's easy to take such an attitude for granted, in the 1960s, the mere presence of such a multiethnic cast spoke volumes on its own. Its greatest achievements were the episodes where the show used analogies to address social concerns, such as the Cold War (with the Klingons as proxies) or racism (through ridiculous black-and-white makeup).
The Next Generation's focus was on an idealistic message of peaceful cooperation — perhaps an over-idealistic message, considering the near-total lack of internal conflict, particularly in early episodes. Regardless of the problems with storytelling, however, TNG's strength was the hope of completely moving beyond the conflicts which have so defined our recent (and not-so-recent) history. One classic example would be the fan favorite "Yesterday's Enterprise", where a single act of mercy and kindness averts twenty years of bloody warfare.
Deep Space Nine's message was much less idealistic, but no less apt. Although TNG's aura of total peace was left behind, it was hardly forgotten, and the characters had to deal with the necessity of compromising one's values in the face of losing everything. The perfect example of this perspective would be the unforgettable "In the Pale Moonlight", where Sisko becomes an accessory to murder, an act which probably saved the entire Federation. We know that Sisko's actions were wrong... but they were still necessary. (Contrast that to today's world, where many governments or their agents wouldn't give a second thought to such an act.)
It's much more difficult to find a central theme in Voyager or Enterprise, unfortunately. If I were to venture a guess, I'd say that Voyager's central theme was family, and unity in the face of adversity. Unfortunately, the series didn't touch on these themes nearly as often as they could have. But when they did, the conflict and the resolution really showed how diverse groups could grow together to become one — like the nail-biting cliffhanger "Scorpion" showed, through the conflict and reconciliation between Janeway and Chakotay.
But Enterprise... what is Enterprise's message? Is it espousing the qualities of boldness and exploration in the face of repression by pointy-eared pinheads? Is it about acting like naive, clueless morons who won't ask for help when it's needed? For its first two seasons, it seemed as if Trek's fifth series meandered about without developing a clear message.
Fortunately, it seems that the writers realized that problem, too, and rectified it resoundingly in the show's third season, with the Xindi arc. Although I couldn't give the season as a whole a complete thumbs-up, I still think that the season was still a success. This was in no small part because of the fact that the show finally discovered its place and its message, at least for the short term — that vengeance is not the answer. The devastating Xindi attack in "The Expanse" was quite obviously a parallel to the September 11 terrorist attacks, and although the parallels between reality and the series' developments effectively ended after that, Enterprise's message was no less appropriate afterwards.
What made the show's third season so great was the discovery, in phases, of how the Xindi could eventually become great allies of Humanity. It started in "The Shipment", with Archer meeting one lone Xindi who, like him, was willing to trust an apparent enemy. Over time, one of the most prominent villains of the season, Degra, turned into a crucial ally — again, one who was willing to take a chance and trust someone who he'd been told was an enemy. This message can easily transfer into the real world, where it's important for people to realize that violence can not always be the appropriate solution to a conflict.
This message is most apparent to the audience through Trip, the eternally likable engineer, who, despite a devastating personal loss, manages to come to terms (to a certain extent) with the man responsible for the loss. As so many people fail to realize today, it's understanding that will ultimately win the so-called "War on Terror"... not the military or the intelligence agencies. Only when we can resolve our differences can the conflict truly be resolved.
It remains to be seen how Enterprise will continue this theme in its upcoming fourth season. But like all Trek fans, I've got hope. I've got faith... of the heart. (Ahem. Sorry.)