What is unusual about the following words: revive, banana, grammar, voodoo, assess, potato, dresser, uneven?

“They all contain double letters”

While some of the words do contain double letters, not all of them do. For example, 'revive' and 'potato' do not have double letters.

“They are all spelled the same forwards and backwards”

This is incorrect because, although some words like 'banana' have symmetry, none of the words listed are palindromes when spelled in their entirety.

“They all have an even number of letters”

This guess is incorrect because not all the words have an even number of letters; for instance, 'voodoo' has six letters (even) while 'potato' has six (even) but 'banana' has six letters (even) and 'dresser' has six letters (even), so this does not apply to all.