Lordaeron - 1x server, bosses have their difficulty tuned up, restricted Marketplace / item store (details on item store: http://forum.warmane.com/showthread.php?t=310303)
Frostmourne - 7x progression server (progresses through each WotLK patch, each season lasts a year), bosses tuned similarly to Lordaeron, two raid resets in a week, normal Marketplace. Characters moved to Icecrown every season end. Details: http://forum.warmane.com/showthread.php?t=415559
Icecrown - 7x server, bosses are tuned normally according to retail 3.3.5a, normal Marketplace
Population: Icecrown > Frostmourne > Lordaeron.
Icecrown tends to reach the 12k cap on peak hours, which means queues (Premium accounts - aka a 10 coin donation minimum - can bypass queues). It's not as big a concern on Frostmourne (they reach the cap very rarely iirc) and is generally a non-issue for Lordaeron.
As https://www.warmane.com/information details, Horde generally outnumbers Alliance across the board, though still to acceptable margins - especially considering that Horde tends to have more PvE players in general from what I know.