Actually, there was a change in editions, it just wasn't written on the cover, like they do nowadays. AD&D 2 Edition was the first one to stop using "edition" in the usual sense.
And if you want to get technical, they actually never changed the meanings, as these "2nd Edition", "3rd Edition"...
Non-human PCs; the power creep; the use of non-standard dice; most classes except the three original ones, the thief in particular; nine instead of three alignments; the higher level character; spells past 5th level; experience for defeating monsters; weapons causing different damages; most of...
Not much, but:
The relativelly fast combat; Vancian magic; the implied don't-pull-punches attitude; experience coming mostly from gold; and the non-customizable classes.