This is the effect of binge-watching (i.e. watching over days/months/weeks, but at will anyway) vs. watching weekly over the course of several years with huge breaks between series.
Lost’s big problem was that it created all these exciting mysteries, and then, slowly, over the course of years, either failed to resolve them, or resolved them in very let-down-y ways, whilst introducing new, and far less interesting mysteries. It also seemed, for the first two, seasons and arguably the third, like it was basically science-fantasy, before ripping off it’s mask to reveal it was mere mysticism and fantasy all along - except that mask didn’t fully come off until S5 or S6. Even if you didn’t watch the original at all, I strongly suspect from the dialogue around the ending, you detected the show was of a basically mystical/supernatural bent, and I think this probably coloured your viewing, too - you almost certainly did not expect a science fantasy show, but many people did - fantasy shows were usually about flash supernatural stuff at that point, and had very clear messaging about their content (c.f. Charmed etc.). Only after Lost did the “Is it SF or is it fantasy?!” thing become common (though arguably X-Files started it).
You, who didn’t watch, seem to see the anger as coming out of nowhere, or just being directed at the end, but that’s completely wrong. The anger was building up for several seasons. People were losing patience with the show, but the writers had created sufficiently compelling characters that people wanted to find out what happened to them, even as everything around those characters gradually got shittier and shittier.
What you may or may not remember was the frantic theorizing between episodes and seasons about what was going on. Not just between people, not just on the internet, but in the media too. We see various papers and sites trading off GoT theories even after GoT is done, but it was, if anything, more of a mass-culture phenomenon than GoT theories are. There were late-middle-aged ladies in my office discussing whether nanites were a plausible explanation for the black smoke - people who I’m pretty sure had never even said the word “nanite” before in their lives.
And that was another deadly problem - many fan theories were very good. Not in the usual fanwank “here’s how it should have ended” shitty way, but actually really cool theories which explained stuff, made sense, and made the show seem more awesome. Except, pretty much none of those theories turned out to be correct.
This was the first time this had really happened, too. There’d never been a mass-audience show which had an 18-49 demographic all theorising about what was going on in a huge way before. By the time GoT came along this was old hat, but then we didn’t know to expect disappointment. We thought that we were going on an extremely exciting ride and trusted, however stupidly, JJ and friends to drive us somewhere cool. Except instead we ended up at a slightly second-rate hippy retreat.
I mean what I remember being particularly upsetting for me was the abrupt change from “Crichton-esque but with more weirdness and philosophy” to “basically a cut-rate take on The Stand” (i.e. sub-Stephen King), which was most obviously signaled by the appearance of Jacob and the Man in Black, both of whom were infinitely punchable, particularly Jacob, and the writers did not seem to get how punchable they were.
The ending itself also caused a big problem, because the way it was shot, with the footage of the crashed plane at the end (which may have been removed again in the Amazon version or whatever - it was just there to fill space to prevent cutting from the real end straight to adverts, which the show-runners thought might piss people off), lead people to think no-one had survived, not that the bit at the end was just a post-life thing. I understood it correctly but the people most irritated did not, and explaining it to them understandably did not make them much less irritated.
I actually agree that it’s not a terrible ending at all. For me all the biggest problems come from boneheaded stuff in S5 and S6, particularly S6. So by the time I got to that ending, an awful lot was resting on it. And it wasn’t going enough to make me go “Hey that was all worth it!”, instead it made me feel like I’d basically wasted my time for two seasons. A bunch of people I knew did stop watching in S5, and I felt like I should have joined them.
For all that, my memories of S1-3 are very positive, and the show still gets quoted by my friends and I. “I will ambush them.” is a favourite. Good old Sayid.
EDIT - Note re: punchability of Jacob. I read that he was directly inspired by Aslan. Which pretty much explains everything about him. Imagine if Aslan, in the books, was a smug blonde dude, not a lion. How punchable would he be? Extremely punchable, I say, because he ain’t no humble Jesus type.
Also this is relevant: https://filmschoolrejects.com/evolution-of-the-mystery-box/
This is part of what I’m saying about context - this was the first time most people had really bumped into this concept. Fifteen years later, it’s how a huge chunk of storytelling works, and we’re largely used to it, so expecting yourself to have the same reaction as the cave-people of 2004 is not realistic. Further I should note that the show itself encouraged people to solve the mysteries, as did the the writers, who also kept saying they did have answers for the mysteries and so on (before abruptly admitting in a later season that some of this stuff they were just throwing out there to see what worked, and had no long-term plans for. It didn’t help that Lindlehof, for example, changed his story on this multiple times).