Learn Italian: Niente è tanto sgradevole quanto una stanza disordinata. - Nothing is as unpleasant as an untidy room.

The prefix s- is often added to adjectives or verbs starting with a consonant to give them an opposite or negative meaning. Here, it attaches to gradevole (pleasant) to make sgradevole (unpleasant).

Why does disordinata use the prefix dis- instead of s-?

The prefix dis- works exactly like s- to create an opposite meaning. While s- is very common before consonants, dis- is usually required when the base word starts with a vowel, like ordinata (tidy), making it disordinata (untidy).

Why do we use tanto ... quanto here instead of così ... come?

Both tanto ... quanto and così ... come are used to say "as [adjective] as" in comparisons of equality. You can use either one here! Saying così sgradevole come would mean the exact same thing.

Doesn't niente usually require a double negative, like non è niente?