I like using adjectives but I often hear they make for weak descriptions. What's so bad about them?

Certainly adjectives, which modify nouns, can be essential. If your character comes upon a dead skunk, it will be important to include that adjective—dead—lest the reader expect that skunk to run and hide or, worse, feel threatened and spray.

However, nouns and verbs are the foundation of writing and you should choose them carefully, rather than slathering on adjectives (or adverbs, which modify verbs). A man could “walk slowly,” but it would be stronger to say “he stalked” or “he plodded.” A garden of “bright flowers” isn’t nearly as vivid as one with “irises and lilacs.” Note the difference between the first sentence, laden with adjectives and adverbs, and the second, which depends upon strong nouns and verbs:

The large, sleek space shuttle went through the clouds precisely.
The rocket pierced the clouds.

The noun and the verb in the second sentence accomplish more. “Rocket” creates a distinct image and carries the force of such a machine. The verb “pierced” implies the precision that’s directly stated in the first sentence and avoids the blasé verb “went through.”

Once you become meticulous in choosing your nouns and verbs, you can then save the adjectives and adverbs for the moments when you really need them.

Perhaps Mark Twain said it best in a letter urging another writer to revise: “Go to work and revamp or rewrite it. God only exhibits his thunder and lightning at intervals, and so they always command attention. These are God's adjectives. You thunder and lightning too much; the reader ceases to get under the bed, by and by.”