A commentary for 3 hours is not easy, but this one was good value. A lot of it was the director, Tommy Lee Wallace, musing on varying things, including the structure of the first half, which he liked a great deal, and several of the bits he didn't like much at all (which he did tend to attribute back to the novel, some of which was fair -- or at least debatable -- some of which I thought was less so. The inclusion of Bill's wife, for example, I felt gave a necessary incursion of 'real life' adult responsibility into proceedings).
Then there's the spider. I know people who complain about the spider in the novel, thinking it a bit of a cop-out. I don't really agree, though in part that's due to things outside the novel, such as the 12 animal guardians at the vertices of the beams -- not an irrelevant link, given the inclusion of the Turtle in It. In the novel it also remains on the metaphysical level, which helps. In the movie it doesn't, and Tommy Lee talks about how he knew it was a problem and tried to get through it nonetheless. I think he did pretty well -- the spider is a bit silly, and certainly nowhere near as effective as the scene where the alien lights pass over the kids down the sewers, for example. And though it does reduce the stakes of the movie somewhat, it allows a reasonable visceral ending which is satisfying enough. Could it have been a bit more mataphysical on screen? I suspect so, but I also think the show was already breaking enough ground with a reasonably faithful adaptation of Stephen King for television.