I was 20 when I watched it for the first time. It struck me with it's depth, sense of humor, mysticism. After 20 years I got DVD edition, and could not stop watching. After 110 episodes I put the first DVD to the player again. After the second turn I did it again. And again. It is unbelievable how Cicely became my hometown, how it's inhabitants became my friends. Never before and never later have I felt such a bond with fictional characters.
Of course, I can see very poorly built Maggie/Joel interactions, I can see occasional pretension, some clichés (or rather some things which NOW seem to be cliché, but were original back in 1990), I can see sometimes desperate attempts to tie some loose ends of the screenplay.
But all that doesn't matter because of overwhelming climate of peace, friendship, love, pensiveness, with ingenious, extremely surprising ideas from the edge of poetry, humanity, philosophy and simple life. It is the ONLY show ever that really makes you THINK and feel. The only which gives you as deep experiences. The only which makes me laugh and cry (almost).
The bad thing is, however, that present viewers can not appreciate it. Why? No firearms. No lasers. No rapes. No murderers. No fights. No blood on the walls. No naked chicks. No intestinal gases. No swearing. No sport cars. Ergo, it is boring.
By the way (spoilers): The series were aired 1990-95. In 1994 the "Friends" show launched. If you know both of them well enough you will see that "Friends" are almost "Northern Exposure" remake, but as just simple comedy (with intestinal gases included), of much lower intellectual level. There are hundreds of similarities, not obvious but visible (e.g. one of the friends is ordained and performs marriages (Chris/Joey), Rick/Phoebe's mom after death comes back as a dog/cat, Ruth-Anne cannot learn Italian - Joey cannot learn French, Rick/Ross die by a blimp/satellite, and so on - almost every little detail in Friends is copied from the Northern Exposure if you pay attention).
Anyway, if you are able to think and to feel, if you want to experience something really peaceful and deep - it is a "must see".