Back when Door to Nothingness was first printed, I saw it as a novelty card. Sure, 5-colour decks had existed before (as evidenced by cards like Coalition Victory), but there weren't that many outside of Sliver decks. As a result, I never saw it played. So, what's the big change? 3 words: colourless mana fixing. One of the biggest problems with running this card is the fact that you need 2 mana of each colour, something which wasn't very easy to do back in the day but is easier to do today.
Cards like the Obelisks from Shards of Alara, Alloy Myr, and Khalni Gem let players have multiple colours in their decks without having to worry about having the right mana out to play it. Sure, cards like the Cameos and Gilded Lotus were printed before or even in the same block, but there was a problem with the cards before this set: they weren't widely used. Gilded Lotus, for example, was a rare that was really expensive price-wise and usually only came out in the late game whereas the Obelisks and the Mana Myr were cheap and common. The Cameos only produced 2 colours, so you would need a lot of of them to make this card work while Khalni Gem make 2 mana so you'll only need 3 in a 2-colour deck. There may have been cards that produced multiple colours of mana before Fifth Dawn, but more efficient and wider colourless sources came afterwards.
So, that's it. If you have any suggestions for more cards that I should compare when it was first printed to the latest printing, feel free to let me know in the comments below.