How does cross realm fix anything though?
If Lord has 5 tanks, 5 healers, and 50 dps queue up at the same time, you'll have 5 groups with 35 dps left waiting.
If Icecrown has 5 tanks, 5 healers, and 50 dps queue up at the same time, you'll have 5 groups with 35 dps left waiting.
Now mash those two together, you have 10 groups with 70 dps left waiting.
Random numbers but the basic concept is;
A shortage of tanks or healers is a shortage of tanks or healers, whether it's individual servers or cross realm. Implementing cross realm doesn't magically add more tanks or healers to the game. Sure, they're added to the pool of available players in queue, but there's also a proportionate amount of dps also thrown in that pool.