I had a long smurfing reply here but vBulletin ate it, so if something isn't coherent that's probably why.
People stopped watching the show because they stopped being able to follow its convoluted plot threads. Many of them have admitted as much in this very thread. That's fine.
Lost is a show that demands lots of attention and it's not a slight on anyone who isn't willing to put in that attention if they can't follow it. I don't think it's really the writers' fault if some viewers get tired of putting the effort in either though. A show like
Lost demands a lot of emotional energy and a plot that demanding isn't to everyone's taste, just as science fiction in general isn't to everyone's taste, or as metal and hip-hop aren't to everyone's taste. Long-running mystery shows take a special kind of audience.
Moreover, it's pretty clear that the convoluted plot was by design. Damon Lindelof has
acknowledged that Robert Anton Wilson was one of his primary influences, to the extent that the number 23 was one of The Numbers purely because of the 23 Enigma that appears in
Illuminatus! and other works by Wilson. Another concept of Wilson's is "Operation Mindsmurf".
Illuminatus! was very deliberately constructed as a work that would make readers question reality by presenting plot twists that would recontexualise everything that had previously been presented in the plot.
Lost doesn't go quite to the extent that
Illuminatus! does of having the characters actually realise they are characters in a book, but it does pretty clearly up the scale of the conflict in a manner that recontexualises the previous plot: Others vs. Lostaways, Ben vs. Widmore, Jacob vs. Man in Black, etc.
That said,
most (not all, for sure, but
most) of the important plot elements revealed from the very beginning are in fact given explanations, whether it's in the show itself or outside of it. The polar bears are explained. The smoke monster is explained. The plane crash is explained. People may not
like those explanations, but that's a whole different problem from not having been explained at all.
Questions like "What is the nature of the island?" were left unanswered, though. I'm not sure why anyone ever expected them to be, really. From what I remember, Lindelof and Cuse had been saying pretty much from the time I started paying attention to their interviews (I think starting with the fourth season?) that trying to explain the Force was one of the dumbest things
Star Wars ever did, and that there was much more drama and mystery to the saga when you didn't know things like midichlorians existed. Explaining what gave the island its magic would have been taking the mystery out of it.
This isn't to say that everything the show did was to my liking - it's never been satisfactorily explained, for example, while Michael's actions mean he can't move on while Ben's do, and there are a few minor plot elements like the reason for Libby's stay in a mental hospital that I feel weren't given satisfactory explanations, but most of these latter issues were background questions. Lostpedia has a list of
unanswered questions, and most of them either are midichlorians-type questions ("why can't the smoke monster pass through the grey ash") or are pretty minor and don't really affect the plot ("Did Ben kill or order the death of the real Henry Gale?", "Why was Libby in Australia, and why was she leaving?", "Where did the nickname 'Hurley' come from?", etc.) There are a couple of things that could qualify as plot holes (for example, Rousseau not recognising Ben, though charitably to the writers we can assume that a person's memory of a face after sixteen years might have disintegrated, even if that person did kidnap their daughter), but that's to be expected in a show of this scope. I also feel the problems with the show are often vastly overstated by its detractors, and overall it seems to explain a lot more of itself than BSG did.