For 6 years lived, ate, slept and breathed Lost. I was utterly hooked and spent stupid amounts of time on the Fuselage website posting and debating theories. I remember being utterly over-invested in the theory that Charles Widmore was his own grandfather.
The problem with the ending of Lost, IMO, is that they spent five years building really intricate plots and mysteries - so many characters linked in interesting ways, so many breadcrumbs laid, tiny references, easter eggs and call-backs, then season 6 happened. We all thought that answers would be revealed throughout the season but instead we got a completely new mystery - the flash-sideways plot, which came out of nowhere and took up time that could have been better spent continuing the ongoing plotlines which had been abandoned in favour of this new format. Then the finale was effectively an answer to the question of what the flash-sideways was about, while not really feeling like a finale/answer to the series as a whole.
Don't get me wrong. There were some wonderful points of the finale - I loved Hurley becoming the island's protector and naming Ben as his number 2. Then Ben reusing to enter the church - extremely poignant. In many ways it worked well but in others it was unsatisfying. There were clearly some issues with filming and communication with the actors too as over the years, so many of them have come out saying that they didn't understand the ending, despite being in it, and some even perpetuating the idea that they were all dead the whole time (it was spelled out so clearly that this wasn't the case).
The main problem that Lost had though, was that it was never going to be a casual-viewing show. In a way, it was ahead of its time in the way that it used the internet and fledgling social media to create a community and further the mysteries. If released now, I could imagine it working well as a binge-worthy Netflix series but at the time, few folk bothered tuning in week after week and thought they could catch-up. The polar bears were explained fairly early on, but pretty subtly yet a lot of people seem annoyed that the finale didn't address this.
I do believe Lost to have been an exceptional series and will willingly die on that hill. Scenes such as the truth being revealed about "Henry Gale", "We're gonna have to take the boy" in the season 1 finale", Alex's execution and Ben's reaction, "Not Penny's Boat", THAT flash-forward reveal and, as mentioned above, Ben alone outside the church were utterly astonishing on first viewing.
When it ended, yet The Fuselage remained, I was among those who were hoping for Lost 2 to happen in 15-20 years with the plan that it would follow the children, Aaron, Charlie, Ji Yeon etc, being drawn to the island, possibly with Walt as the enigmatic Locke-type character. I still think there is mileage in this!