While certainly different, I would not call them completely different. We have been able to play more or less in the same styles (the ones that we prefer) for more than 35-40 years whatever the edition. Some were easier to use than others, some steered our game in particular directions, but it was still unmistakeable D&D.
But this might be because we have a heavily story-oriented roleplaying mode, I understand that if you play more technically (which is as good a style if you prefer it), the editions of the game seem more different to you.