Learn Icelandic: Þetta er verra hótel.

Comparative adjectives must match the gender of the noun they describe. Because hótel is a neuter noun, we use the neuter form verra. If you were talking about a masculine or feminine noun, you would use verri instead.

Why do we use þetta here instead of þessi?

Þetta means "this" for neuter things. Since the noun it introduces (hótel) is a neuter word, the pronoun must match its gender. If you were pointing out a masculine or feminine thing, you would use þessi.

Why is there no word for "a" in this sentence?

Icelandic doesn't have a separate word for "a" or "an". To say "a hotel", you just use the bare noun hótel. If you wanted to say "the worse hotel", you would add the "the" ending to the noun: verra hótelið.