So I found a little info in the forums about each realm such as this:
Q: What are the rates of all of the realms?
A: Lordaeron is x1 across the board. Icecrown, Frostmourne, Outland and Frostwolf are x7 XP and x3 everything else. Blackrock is instant-max-level pvp-only.
and
What are the realm's rates?
[wotlk] Lordaeron: x1
[wotlk] Frostmourne: x7 experience, x3 gold rate, weapon skills, profession and reputation rates
[wotlk] Icecrown: x7 experience, x3 gold rate, weapon skills, profession and reputation rates
[wotlk] Blackrock: Instant 80 (only PvP)
[mop] Frostwolf: x7 experience, x3 gold rate, weapon skills, profession and reputation rates
But is there anything else different? Why is Icecrown so packed compared to Frostmourne which seems to be the same rates?
Btw:
Lordaeron: it is x1 realm with harder pve content (Bosses of Wotlk raids have more hp, more dmg and shorter berserker timer). It was a "progressive realm" but, since it has bene launched on september 2015, it is stacked at ICC-RS since many years
(no gear on Coins-Shop)
Frostmourne: is a "progressive-seasonal" realm. Which means every year it get resetted for a fresh new start. It emulate 3.0 patch and progress to next the patch every 3 months
Icecrown: it is the older realm, since it is the result of the merge of Deathwing+Ragnaros realms. (that is why is the most populated, it is stuck at ICC-RS since always)
Blackrock: it is the old Wotlk-Arena Torunamnet realm . Instant max lvl, only pvp. You can do Duels, BGs, Arenas (you will face enemy from ice/lord as well in bg/arena)