First, as Jacob pointed out, Shelob is her own undoing. It's telling that Shelob's body is described as her "precious flesh," because appetites and lust is what defined the monster in the last chapter as well. Shelob seeks to crush Sam with her body, her flesh, and at the same time her flesh is her own undoing. It's a subtle but scathing rebuke of hedonistic pursuits. (More has been said elsewhere about how Tolkien chose to clothe this theme in gendered terms; it is somewhat unquieting that this bloated, ghastly evil is described as female, but perhaps it's merely thematic; male Sauron, female Shelob.)
On to Sam's choices. After he realizes that he made the "wrong" choice later in the chapter, Sam castigates himself by stating that his choice was informed by his lack of hope. An accurate, if depressing sentiment, in my opinion. Sam revealed in previous chapters that he didn't actually have much hope that the Quest would succeed; he was just seeing it through to the end with his master. Here, that lack of hope spills over onto his assessment of the situation: Frodo is cold and non-responsive, without breath or pulse; by all standards he is dead. However, Sam is no stranger to supernatural forces. With a little more hope in him, would he have realized that there was a chance that Frodo wasn't dead, but just poisoned? In any case, Tolkien has Sam conveniently lose his head and leave Frodo lying there in the middle of the path for anyone to find, instead of moving him to the side or among some rocks and covering him with his cloak. (Of course, if he had done that, the orcs wouldn't have found Frodo, and Sam wouldn't have realized that Frodo was still alive. So it's all for the best.) It's interesting just how casually Sam takes the Ring, but then immediately feels the weight and burden upon him. "Somewhere, an Eye was searching for him," the text relates, and it makes the reader wonder whether Sauron can feel the Ring being used by another. Sam's innate power is so minuscule, however, so even if Sauron knew the Ring was being used, surely he couldn't sense where it was taking place.
And finally, the orcs. There's a lot to process in this dialogue. A couple of things stood out to me this time. First, either the orcs have extremely long institutional memories, or they're immortal, like the elves. Shagrat and Gorbag are talking about "old times" and the "Great Siege" as if they occurred yesterday, not (literally) thousands of years in the past. I suppose it makes sense that the orcs, made as twisted mockeries or perhaps an offshoot from the elves, would have long lives. I guess I never really considered it before. Second, we revisit the theme of discontent in Sauron's ranks. Shagrat makes it clear that he believes he has spies for the higher-ups among his own people; talk about a police state. And Shagrat and Gorbag have a healthy mutual distrust (that will blossom into violence) for each other, their own men, the Nazgul, and even Sauron himself ("the Biggest Boss" as they call him). They're anxious to take the first opportunity they can get to bolt away from the Enemy's control and set up shop on their own far away from any "Bosses." Part of this discontent of course comes from the aforementioned police state and hypermilitarized society the orcs seems to be steeped in (Tolkien, as a veteran, no doubt parroted the orcs' military lingo off of what he heard in the military himself). And part of it comes from the fact that the orcs are all too aware that they are just expendable drones who could be shoved into the meat grinder at any time. They're unsettled by the magical means of communication that pass information they are not privy to (see their conversation about how "the messages go through quicker than anything could fly") and realize they're in a damned if they do, damned if they don't situation. If the "good guys" win, they know they're just as screwed as if Sauron ends up triumphing. What a life. I love the insight into how the bad guys operate. This is more nuanced, I feel, than the petty squabbling we saw between the Isengard and Mordor orcs in the last book.
To sum up, this is a great chapter that provides a killer cliffhanger and excellent setup for the next book. I remember being stunned when I read this for the first time as a kid. The explosive last few chapters made up, in my opinion, for the plodding pace of the rest of Book IV.
I'm not as excited, however, to revisit Jackson's "Two Towers" film. It has been, historically, my least favorite of the three. Bring on the hyena-wargs and the never-ending Helm's Deep battle scenes.