You are presented with five groups of five words. All of the letters in each word share a common trait, except for one. The rule governing which letter is different is consistent within each group.
The unique letters in each group spell a word. Applying the group's rule to that word produces a single meta-letter. Use the five meta-letters to find the meta-word which answers the title question.
asia
maui
scratch
eerie
auto
blacklight
foreshadow
dynasty
napalm
roleplaying
gasped
deadbeat
horticulturist
bagpipe
decoded
corrupt
snowplow
nervous
yawn
challengeable
perpetrator
without
falafel
equator
tidy
Bonus: What is the meta-meta-letter?