1. What do the following words have in common, other than starting with A and having three letters?

ACT
AIR
ALE
ART

2. What do the following words have in common?

ACOUSTIC
ANALOG
CLOTH
ROTARY

Answers:

1. Each word can be preceded by IMP to make a new word:

IMPACT
IMPAIR
IMPALE
IMPART 

2. They are words that describe things that once needed no description. All guitars used to be  acoustic, all clocks and watches anaglog, all diapers cloth, and all phones were rotary. With the popularity of electric guitars, digital clocks and watches, plastic diapers, and cell phones, these words have to be used to distinguish the old from the new.