From what I've seen, the general fan reaction to this book was uncannily similar to that of the deranged fan in Stephen King's Misery — Adams, wanting to move on to something else, kills off his Hitchhiker characters, and the fans froth at the mouth. Part of it's the typical "you may have created this but I like it a whole lot and so therefore it's mine" fan thing, but there seems to be more to it than that: reading some of the reader reviews, I couldn't help but be reminded of the most annoying person on the MUD where I hang out, a thoroughly infantile sort prone to sententious remarks like, "Megatron may have been the leader of the Decepticons, but let's not forget that handguns don't need to turn into giant robots to be dangerous" and similar. This person also runs screaming from any narrative that's remotely downbeat, no matter how good, so no The Sweet Hereafter ("Depressing!"), no Hamlet ("Depressing!"), etc. Guess which word pops up over and over again in these reviews? I've heard the rationale — their lives are so miserable that they can't stand the slightest bit of darkness in fiction, which they turn to not to understand life but to escape from it. I'd quote Anais Nin here — "we see things not as they are, but as we are" — but these people would probably find that sentiment depressing.
What really boggles me is how the first four books got through their filters. "I don't know why [Adams] wrote this book," one wrote. "He took a wonderful quirky set of stories and turned them into a moral that no one wants to here [sic]: 'The evil, stupid bureaucrats always win.'" Er, excuse me? And that wasn't the moral of Arthur's house and eventually the Earth getting demolished to open the first book? That wasn't the moral of Golgafrincham? Of just about every page of everything Adams ever wrote? I mean, it's not like it's the most varied oeuvre you'll find: it's pretty much a catalog of all the different ways bureaucratic thinking can lead to ludicrous outcomes, with the parallel thesis that bureaucracy is pretty much the defining essence of humanity. Of the freaking universe, for crying out loud. It's not like this is a new thing.
Mostly Harmless has some other observations, however. There's a long sequence that's basically about the experience of going back to one of your old hangouts to find that it's totally different now, no longer your place; there's some family drama, with comedy of course but not played strictly or even primarily for laughs; and then there's Chapter 2, which is probably the best bit and not just because it opens with a full page spelling out exactly the extent to which New York weather sucks. Chapter 2 could be a short story all its own, not just because of its theme of alternate selves — "if I'd made one different seemingly inconsequential choice, I'd be living this whole different life," not exactly the most original theme, but interesting in that we know about that different life from four previous books — but because of its very trenchant observation about heuristics. See, our Tricia McMillan ran into Zaphod at a party and left with him to go roam the galaxy, but this new one went back for her bag and the ship took off without her. That's not the clever part. The clever part is that now she has this hangup: "if life had taught her anything it was this: Never go back for your bag." Which is a nearly universal thing that humans do — a bee stings the top of your foot and you vow never to wear sandals again, that sort of thing. And then she blows an audition because she didn't go back for her bag, which had her contact lenses in it, without which she couldn't read her script. Later, she reflects that "if there was one thing life had taught her, it was that there are some times when you do not go back for your bag and other times when you do. It had yet to teach her to distinguish between the two types of occasions." And that, I'm saying, is pretty frickin' profound.
One last note. I took the dust jacket off this book since I figured it'd get tattered if I kept it on the book when I put the book in my bag, and as I did, a piece of paper fluttered to the ground. I looked at it and remembered that, yeah, that's where I'd put this thing: a certificate for winning the '98 IF comp, signed by Douglas Adams. It had apparently been fairly difficult for the person in charge of the certificates to get his signature. Now, of course, it is somewhat harder still. Sigh.