I have praised JMS for the things he did before this series, mainly Babylon 5, and this series is no different in that regard. Jeremiah would have been a fabulous arc if, if... well enough with if's.
We have to be grateful for the 2 seasons that aired.
Jeremiah was about the aftermath of a plague of sorts, which killed all adults. Only teenagers or younger were left alive. It is safe to say that the human population was in a state of chaos after that. How could a bunch of children run things?
The answer is, they can't. Jeremiah starts when the main cast is about 30 years old, 15 years after the plague killed all adults. The first season is a very interesting take on the life of the cast in the new world. JMS really knows how to write, it's a good season.
The cast of Jeremiah was all the way excellent and the quality of the show in all things was above average.
Season 2 however was, in my view, the highpoint of the series. In that season there are huge alliances and wars that really impressed me. There are many exciting and climatic episodes in season 2 and those define Jeremiah.
That's more or less the same way JMS did Babylon 5, first season was a slow and composed way of building a solid base for the series, a solid base for the characters so they can be developed later on. In the second season the fireworks start, just like Babylon 5. It's an magnificent way of building a series, works like a charm.
We can only wonder what kind of fireworks and intense drama we would have seen if the series hadn't stopped there. As I said, it's a great shame, but no need to worry about that.
I can safely say that the ending of the series is pretty good, all things considered. Some may consider it a major cliffhanger but to me it was a good place to end the series, unlike many other series which have been cancelled after a major cliffhanger. This one ended well and I have no regrets concerning that.