Some travel subjects teeter on the edge of being appropriate for a travel guide. This page lists a few of these slippery slopes and what to watch for.
Just because a subject is on this page doesn't mean Wikitravellers shouldn't write about the topic. They should just be clear about not slipping down the slope.
It can be really helpful before travelling to a destination to read fiction and non-fiction books about the destination or by writers from there. However, it's probably not appropriate to add lists of general "vacation books" or "airplane books". The slippery slope begins when people start adding any good books. Wikitravel is not a book review site; we should keep lists of books specific to destinations or subjects.
A similar subject is music. You can know a lot about a destination, its people, and its culture by listening to music by local musicians or in the local musical tradition. For example, if you go to North Africa, it'd be great to learn more about Rai music and get a list of good artists and albums to start off with.
The slippery slope comes when we try to list all albums and Rai artists. Or when we start adding in good music to listen to on a train or airplane, or from there go on to listing good music in general. Wikitravel is not a music review site; mentions of music should concentrate on providing cultural touchstones and not just recommending music for its own sake.
Travel topics are another slippery slope. It's probably useful for Disabled travellers, Gay and lesbian travel or Women travellers to have a few articles about specific issues. The danger lies in taking things too far -- into advocacy, complaints, lists of pet peeves, emotional personal essays about particular travellers' particular experiences.
Travel topics should be concise and simple. We want to concentrate on practical information, and not on noodling and speculation.
Travel tips are really a central part of having a travel guide. They are definitely practical information, and they should be featured and supported. Driving in Australia is a great example.
However, there is a subtle slippery slope, namely: isolated travel tips that don't really fit with other tips. A note suggesting to bring a sewing kit with you should go under Packing list, not on its own. Leading downwards to personal anecdotes ("This one time I used banana peels to take stains out of my silk blouse in Lebanon...") and thence into either the dreaded travel essay or into generalized clothing-care tips.
Links to relevant stuff on other websites, e.g. Maps, are a slippery slope. Well meaning contributors need some guidance about this to avoid wasting contributor's effors and causing dissapointing. (see also Wikitravel_talk:Article_templates#External_links)