Breakthrough Hit: It was this game that brought Nintendo into the video game industry, after the company spent an entire century producing other things.
No Export for You: In a bizarre twist, the original arcade versions of Donkey Kong, Donkey Kong Junior, and Donkey Kong 3 (and Mario Bros., plus Popeye as well) were written by Ikegami Tsushinki, a manufacturing company which developed the game for Nintendo. Unfortunately, there was no contract between Ikegami and Nintendo for the source, so Nintendo does not own the original arcade game code. This is why Nintendo cannot sell the arcade version of Donkey Kong for its consoles, but are free to sell ports (i.e., the later NES versions) remakes and updated versions instead.
Considering that a near arcade perfect port exists in Donkey Kong 64, this may also be a reason why that game has not seen a rerelease.
Port Overdosed: Virtually every console and computer of the era had Donkey Kong released for it, and so have many since. Among 1980s arcade games, its omnipresence is surpassed only by Pac-Man.
What Could Have Been: Donkey Kong Jr was possibly going to be more like a traditional sequel where you controlled Mario again, as there are various unused Mario sprites in the code similar to his sprites in the first game.
In a similar manner, sprites of Donkey Kong Jr are in the code for Donkey Kong 3, suggesting he may have been planned to be in that game at some point.