So it was surprising in a sense that I even started to watch Fringe. I think that part of its appeal is that it feels like his first really 'honest' show. It's not a Spy drama with Sci-Fi/Mystery leanings, it's not a disaster-survival show with Sci-Fi/Mystery leanings - it's a straight up Sci-Fi-Mystery show. It may end up with a smaller target audience but you don't feel that it is deliberately courting the mainstream, basically tricking them into watching Sci-Fi when they normally wouldn't consider it. Sadly the same thing seems to have continued with Alcatraz and potentially again with Revolution, where what appears to be a cop show and a post apocalyptic survival drama aren't allowed to stand up on their own without a big mystery behind all of it.
Right from the beginning what really drew me in was John Noble and his deranged portrayal of Walter Bishop. His character was what kept me hooked, with his mixture of insanity, genius and just plain hilarity creating an intriguing character. While the early episodes may have felt a little like the X-Files, he was a constant presence that kept the show interesting. As his personal history was gradually revealed I also found him very believable as someone with a heavy burden upon him emotionally. He even managed to create a strikingly different version of himself in an alternate universe, who was gradually revealed to be more than just 'Evil Walter'.
That said, almost every actor in the show has an alternate universe counterpart and all have done a great job of playing them just different enough to feel like another character. Anna Torv is especially strong as the two versions of Olivia Dunham and I was also impressed by the episode where she is incredibly convincing in portraying her conciousness being overtaken by Leonard Nimoy's character William Bell. I loved how gravelly she managed to make her voice sound in this episode and she seemed to have his mannerisms down just right. Before hearing her and John Noble speaking in their natural accents while being interviewed, I would never have guessed that they are both Australian.
I could go on about almost every character but I'll just highlight one more, Jasika Nicole as Astrid ("Astro? Asterix? Aspirin?") specifically in the last season. The episode where her autistic alternate comes to our universe after the death of her father was particularly effective. It was totally heartbreaking as our Astrid managed to convince her counterpart that she didn't have a good relationship with her father either and that it wasn't down to her autism, only to see her go home to her clearly loving father at the end.
So it seems that I've answered my own question, the characters are probably what keeps me watching. Even Peter Bishop is so much better than you would expect Pacey from Dawson's Creek to be. Given that I can barely remember the events of the first few series, the overall plot took a while to get going - even if the weird problem of the week was usually pretty interesting. There were some great moments as it went on, the revelations of who William Bell was and more detail on what the alternate universe was like. A simple flashback episode was turned into something unforgettable by its '80s styled titles and music. It always came back to the characters though and your desire to see what would happen to them next. That it managed to achieve this and still have you root for them after a memory wiping reboot is pretty impressive.
So for the first time, here I am at the end of a fourth season of a J.J. Abrams show, looking forward to the final season (which didn't even look too certain to begin with). I heard rumours that two endings were possibly filmed depending on whether the series was renewed and I can sort of see how it might have been possible to finish it there. Some of the last episode may have resolved things a little quickly but it was in keeping with the show as a whole, mixing very disturbing scenes and tough decisions for its characters. The hints at what will happen next are quite an exciting prospect, making one last big shake-up and hopefully the show will go out with a bang.