1st Edition was memorable for the adventures.
2nd Edition was memorable for the settings.
3e was memorable for the rules.
4e is memorable for the electronic support.
IMO, that's the big thing that's noticable about this edition but not the others.
I wouldn't go so far as to say 4e is "about" electronic support, but then I wouldn't say any of the previous editions were "about" those other things either.
You have a bit of a point here. If you are looking for something memorable that 4E has added, its hard to top the Character Builder.
I think this is by far the best answer so far, as it addresses the question from the perspective of the designers as opposed to the perspective of the various players - which will differ from table to table depending on how they played the game.
We can say something about what the designers were trying to achieve. We can't really say anything about how the rules were used by the players except to offer opinions about how we personally perceived or use them.
So any way, my take is probably no more than a restatement of what Dausuul said using different words but:
1) 1E was about exploration
2) 2E was about clarity
3) 3E was about consistancy
4) 4E was about balance
What is more interesting that different versions ranked these four in different order. I would say:
1E--Exploration, Balance, Consistency, Clarity
2E--Clarity, Balance, Exploration, Consistency
3E--Consistency, Exploration, Clarity, Balance
4E--Balance, Clarity, Consistency, Exploration
1E was arbitrarily yet effectively balanced. 2E varied in Balance and Consistency, with the supplement material having a negative impact on both. 3E isn't very clear cut, as it focuses on Consistency to the max, far more than the other three. 3E both possesses and lacks Clarity at the same time, as the rules tend to be clear and specific yet the significance and impact of these rules in play are anything but clear. 4E on the other hand while primarily about Balance is also fairly high on Clarity and Consistency, and doesn't focus much on Exploration at all.
If I would rank the systems on a scale of 1-10 in these areas:
1E--Exploration(10) Balance (8) Consistency(4) Clarity(3)
2E--Clarity(8) Balance(7) Exploration(6) Consistency(3)
3E--Consistency(10) Exploration(5) Clarity(6/3--high and low) Balance(2)
4E--Balance(10) Clarity(9) Consistency(7) Exploration(3)
3E is very inconsistent on Clarity, which is why I put in two numbers. Also note that I rank 4E higher in Clarity than 2E.
The weak point of the previous edition tended to be the focus of the next.