Why this happens? Well, it's pretty simple. These Rares drop a few goodies that everyone wants. The most notable ones are the Key to the Palace of Lei Shen, needed for the treasure room single scenario, and the Shan'ze Ritual Stone, needed for the elite group quests.
People keep farming those rares and rage for not receiving any loot or only the Tome of Valor after each kill. Apparently Blizzard clarifies this for us:
Similarly, you can only obtain one Shan'ze Ritual Stone from an Isle of Thunder Rare Elite per week. You'll still be able to collect more from other sources, though.
So basically you should kill these rares once a week only, for the stones and keys. You can kill them constantly though, for the valor points Tome, but waiting for about 40 minutes for a rare to spawn if not very efficient in my opinion. Anyhow, if you are hunting for them you should head over to wowhead and check their guide for these rare mobs.
You might ask yourself now "But how am I going to get 3 stones for the ritual each week?!". Well, it's pretty simple, you just have to be lucky. One will come from the rares, of course, one will come from the treasure room and one or more may come form the Trove of the Thunder King chests.
Now tell us how is your hunting going? Got any keys or stones? Found anything interesting? Let us and others know about your experience with the new Isle of the Thunder.