I'm one of those people currently desperately trying to get the Rare Mounts like Time Lost Proto-Drake, Aeonaxx, etc. For a few days there, I thought I had a chance because the Account-Wide mounts eliminated a ton of competition in the form of people who were hunting them for their alts. Then came the Cross Realm addition... now I have to fight with six other people from six different realms for a really ridiculously rare chance at catching them.
I kindly request that Blizzard removes the Cross-Realm feature from areas where there are rare mounts so that everyone has a fair chance on them on their own realms without having to fight people from other realms for something that comes along a few times a week if you're very lucky.
Other than these specific areas, I have yet to find anything to complain about in regards to the Cross Realm feature.