What makes a supplement not very useful

Generally speaking, what makes a D&D supplement not very useful is when it is too vague.  This vagueness comes in a lack of detail or an amount of detail that leaves the player or DM saying: "I could have made that up, why did I spend my money on this"

If good and useful supplements deliver (1) adventure ideas (2) roleplaying ideas (3) new rules (4) get your imagination going

Not so useful supplements are missing some or all of these elements.

Another dimension of usefulness is how specialized the supplement is.  Obviously if you always play rogue characters, a book about paladins will be less useful.  Similarly if the campaign involves mostly standard dungeon crawls and the supplement is about a place lacking in dungeons to crawl in, that supplement will also be less useful.  In this regard, the gazetteers are mixed.  The following are fairly conventional and will accommodate a standard adventuring party with little difficulty:

Karameikos, Ierendi, Northern Reaches, Minrothrad, Darokin

The following are written for culture or race specific adventuring but can still accommodate outsiders who are visiting fairly easily:

Ylaruam, Alfheim, Rockhome, Five Shires

The following are written for culture or race specific adventuring and some or all members of a standard party will run into difficulty:

Glantri, Thar, Ethengar, Shadow Elves, Atruaghin

The biggest problem that hurts a supplement (any edition) is vagueness.  The language fills up space but does not really convey any information.