This would be a huge quality of life improvement
Posted from the Council — voting happens there; discuss here.
While this is TRUE (it’s a huge QoL improvement) … it’s also something that “makes the world smaller” for the sake of convenience. 
The 1 hour delay for in-game mail delivery of attachments (cash, items, etc.) provides a game useful (if not exactly wanted) bit of friction to the rapid transfer of Stuffs™ from character to character. The habits that Players develop to minimize that friction (only send what you NEED to, plan ahead and stick to your plans) are ones that reward foresight and discipline.
Additionally, the 1 hour mail delivery mechanic actually rewards to creation of alts to play concurrently. Got mail that’ll arrive in an hour but you need to have something to do while waiting for that delivery? Play an alt in the meantime! By encouraging this type of altaholic behavior, the population and variety of alts on the server has more going for it than just what class do you want to Main to 60.
A similar phenomenon happens with in-game transport systems. In games where transportation is instant (usually by teleporter or equivalent), the game world becomes … very small. Players don’t TRAVEL the world, they SKIP OVER IT in order to get where they want to be with the least amount of effort (because: DUH!). But in those systems, the game world invariably SHRINKS from “all the places you COULD go to” down into a mere subset of “only the places you HAVE TO go to” while everything else gets ignored. Even if the zone maps are huge, if you’re not going there/travelling through on your way to somewhere else … the game world “shrinks” down to only the part of it you care about. The rest of the world that you don’t go to/pass through “disappears” from relevance.
A similar thing happens when the 1 hour delay on mail deliveries gets revoked. When mail arrives “instantly” the convenience factor is HIGH, but the “game world” pays the price in lost immersion and opportunities for diversity of play.
It’s a subtle structural point … but Vanilla WoW was developed and released prior to the era of instant communications from anywhere to anywhere (the smartphone revolution hadn’t arrived yet). Throwing away a “friction point” in mail deliveries (1 hour delay) pulls the “era” of Vanilla forward in an anachronistic way of assuming “how things should be” (because they’re that way, NOW).
Huge quality of life improvement … no doubt.
But if implemented, one of the differences that makes Vanilla WoW so immersive as an experience in game play will be lost. Lose too many of those interlocking points and you’ll be playing a different type of game … one in which Instant Gratification Isn’t Fast Enough™ (or words to that effect).
So if you’re looking for a vote … 