1. There's a very fine line between a depressive passive character and a depressive active character. In the original TV series, Marvin the robot is hilarious and touching. In this current movie Marvin is ALMOST hilarious until you realize that he's just WHINING. It's just a minor shading different from the TV actor's performance, but a key distinction. The movie Marvin seems to be saying, "Life sucks, and it makes me sad." The TV robot's attitude was "Life sucks," but he was indignant about it. You could detect a certain active hostility towards the Big Futility of It All, buried under the deadpan delivery. That little touch of resentment gave the actor something to play which resonated much stronger than the current movie version, who seems to be merely commenting and reacting to the fact that Life Sucks.
2. The love story of Arthur and Trillian is, in the books, understated to the point of being almost non-existent, and because of that, unbearably sexy and sweet. I had a vivid dream in my adolescence about the whole unrequitedness of it all. By actually depicting the love story in the movie in the form of cutesy "love dialogue," they somehow managed to destroy a romance that barely existed in the first place. We never really get why Arthur digs Trillian, although there are plenty of reasons seeded into their dialogues in the book, all of which are replaced in the movie by this "she's so pretty/she's the one" kind of hooha.
3. The story necessitates a lot of exposition, but there were a lot of tangents which gave the answers to questions I didn't really care about, while other admittedly confusing things were left undealt-with. The whole beauty of Hitchhiker's is that the narrative pauses so frequently to allow you to ponder some trivial aspect of life in a funny and revealing way. It's a rhythmic device that, again, was a lot better in the TV show.
4. At the risk of being repetitive, spending less money makes movies better. I'm way over the whole looking-cool-and-saying-nothing thing. The TV series looked a bit cheap, but there are many things about cheap that are inherently funny. which is all the movie needed to be considered a comedy.