I understand the wonderful things that come along with being able to group with your friends from other realms: Raiding with them, doing dungeons with them, finishing up dailies with them. Thats great, I love it.
However, with this great achievement, comes a great downfall as well. Many members across realms are using this ability to physically be in another realm that is not their own, to exploit the ability to farm world-wide rares. While this may not seem like a huge problem to some people, to those of us who have spent ages and ages farming Rare Spawns that produce a rare mount, such as the Time Lost Proto Drake, or Aeonaxx, this creates a HUGE problem. The problem is that as you farm these rare spawns with those on your realm, the number of people on your realm who are farming with you AND don't have the mount decreases as those few people on your server catch the spawn. However, with the ability for anyone from any realm to be able to be in your realm (or ANY realm) THIS creates the problem of endless amounts of people farming the same thing you are.
Spots in the Storm Peaks where people from other realms are farming my realm's rare spawns:
And that's just today, in one single area in the zone (frozen lake).
I do not know if this is a "bug" per-say, but it is definitely something that is broken and needs to be fixed.