Most useful information found in Mystara/Known D&D World Gazetteers

The most useful information that is found in some but not all gazetteers are the following:
 
1.  Adventure ideas
2.  Campaign ideas
3.  Interesting character variations
4.  Workable background information
5.  Politics and Rivalries

Adventure ideas:  Many DMs can come up with fairly good adventure ideas on his or her own.  When purchasing a product like a gazetteer most DMs are interested in time-saving opportunities.  Solid adventure ideas with a workable framework are very useful.  These adventures should also be distinct as to setting.  Adventures that are too generic are not as good.

Campaign ideas:  Similarly ideas for how a gazetteer can be used for an extended series of adventures is very useful.  It saves time and makes it easier to build a storyline.  Again these campaigns should be distinct to the particular gazetteer.  A campaign in "The Five Shires" should be different than one in "The Emirates of Ylaruam".

Interesting Character Variations:  Optional rules for the players are almost always interesting.  This most often is variations on character classes or new abilities or new spells.

Workable background information:  Background information about the economy, history, and culture (etc) of the gazetteer should be broad enough to allow some tinkering.  It should not be too broad so as to be too generic.  In other words there should be good "out-of-the-box" functionality but it also should be options for "customization".

Politics and Rivalries:  Well-done political intrigue always makes for better reading.  Good politics and rivalries also make for interesting adventures as the characters work with or against various factions.

The best gazetteers offer adventures and campaigns based on both political intrigue and interesting locations within the gazetteer.