Well, when I think of a D&D world I'd bet money it has the following features:
Druids, rangers, bards, psions, and paladins get access to healing magic. But not wizards.
Spells like Resurrection imply that death need not be
permanent.
Remove disease and other spells allow populations to be healthy compared to real world analogs.
Eternal life and magical healing are for sale by churches.
Polytheism. There are lots of deities and they are actively involved in the lives of their followers.
Sentient zoo: There is an abundance of sentient races beset by a myriad of monsters.
Equality: Males and females have access to any class and there are no ability limits based upon gender.
Cross-class skill penalties result in specialized characters. It’s rare, for example, to find a warrior who can open locks.
Everyone can speak a universal language, Common.
The existence of adventurers hints at a society with lots of individual freedom. Governments seem to tolerate armed bands of vigilante groups.
Magic behaves in very predictable ways, with measurable durations and standard areas of effects. By witnessing a wizard or cleric cast a spell (and then note the effects, such a the amount of damage inflicted) one can estimate the power of the caster.
There are lots of ruins to be explored, as well as vast underground caverns teeming with life.
Characters have easy access to affordable steel weapons and armor, indicating a society with efficient mining and forging techniques.
Land capable of supporting large numbers of predatory animals. Harvests are pretty darn bountiful if your supporting human settlements, bugbear tribes and a number of less-intelligent predators in a given area. Credit: Crazy Ivan
If a spell caster is studying magic from a book he’s definitely a wizard.
If any part of the PHB gets specific about "what things are like in a D&D-world," it's the section that describes what races are available, their favored classes and alignments, and their typical attitudes. There are many settings which help to define themselves by specifically showing how their races are different from the "standard" ones. That implies the races show fundamental assumptions about the setting.
Credit: buzz
Alignment essentially implies a certain moral absolutism. For the most part, we can see that Law, Chaos, Good and Evil are not abstract concepts in the D&D universe. They're tangible and detectable, hence a character's ability to "align" themselves with one or more of them. Thanks again, Buzz.
The economy is coin based, which stands in contrast to real world feudal era, which relied heavily upon barter.
All monks know martial arts.
D) or hate it.
