Already since the 3rd century BC in Alexandria, which became the center of ancient Hellenistic civilization, the works written by Attic authors of the classical period became to be considered as canonical, lexics and grammar of it were recommended as the normal leterature Greek. In the next centuries Attic was several times proclaimed as the only Greek official language, which only strengthened the conservation of the language.
Now the main features which distinguished Attic from other Hellenic tongues:
a) the Common Hellenic long á was preserved after r, e, i, also sometimes after long vowels (e.g. chórá - a country)
b) the aspiration was also preserved (hosos - what number?, Ionic osos)
c) an Ionic preposition and prefix sun became xun in Attic
d) the rs combination was assimilated to rr (e.g. arrén - masculine, Ionic arsén)
e) the Attic vocabulary, as already Ancient authors notices, was rich with borrowings from barbarian tongues, other Hellenic languages; historians call it sometimes a "mixed language".