I enjoy using heirlooms for my alts, but would really like to be able to use them on other characters on different realms within the same Battle.net Account. Since there are limitations with the in-game mail system, perhaps a different approach is needed.
What they should do is make vendor purchases account wide so if you buy an heirloom on one character, it "unlocks" for all other characters linked to that account. Therefore, a newly created character can simply buy whatever heirlooms have already been bought on one character linked to the account already. Each item that has been bought can have an "already unlocked" tool tip so you can easily remember which you have unlocked. This takes away the "transferring by mail" problem and places less stress on the email system.
In order to implement this system, they should make the following changes:
1) Make ALL heirlooms purchasable using gold
2) Change ALL heirlooms to 1-85 without the need to upgrade
3) Place Heirloom Vendors in a more obvious, easily-accessible location