These are a relatively new things for DotA in the last few years that I marvel over at how smart they are:
A) The introduction of quelling blade and poor man's shield, which have closed the gap of ranged vs. melee heroes, making even more heroes viable.
B) Re-introduction of combat mobility via force staff. This effectively addressed all the problems of a blink dagger, but it isn't simply a "nerfed version of blink dagger". Players were given an option with this item; usable on others. This deepens what this item is capable of.
C) Backdoor protection, which by design is not a full protection, but meant to stop one player from backdooring, giving more emphasis on team effort.
D) Arcane boots. This item alone probably increased the amount of mana usage per game by easily double, and making th early-mid game more explosive.
E) New buyback formula. Buyback was always there as an option, but it was never really part of the game. And now it is. Buybacks are game changing, and what more can you say?
F) Smoke. Perhaps one of the greatest concept to exist in the ARTS genre ever. It effectively addresses the strength of vision in a smart, but not overtly done manner. Promotes aggressiveness and just generally opens up more doors for everything.
All of these things are so smart, so today, and so now. I know there are a lot of people who feel that DotA is an archaic game with archaic design principles. I agree and disagree. I won't go into details here, but this is a subject I'd like to discuss some time in the future.
DotA is still experimenting and pushing the boundaries of what ARTS are truly capable of.