I did a research to figure out what exactly mean the “high”, “medium” and “low” status of the servers in World of Warcraft. Here’s the result:

Total Players (Horde+Alliance):
3500+ -> Queues
3500-1500 -> High
1500-500 -> Medium
500-0 -> Low

On servers like Archimonde and Blackrock the Alliance/Horde population is even. On Frostwolf the Alliance doubles the Horde. The norm seems to lean toward the Frostwolf example. On the PvE servers it can be even worst with the Alliance being three times the population of the Horde and more.

The races for the Horde are in this order of popularity: Undead > Tauren > Troll > Orc (with orcs and trolls at about the same level and swapping at times)
The races for the Alliance are in this order of popularity: Night Elf > Human > Dwarf > Gnome (with humans and elves with a wide margin and swapping at times)

The classes for the Horde are in this order of popularity: Shaman > Rogue > Warrior > Hunter > Mage > Warlock > Priest > Druid (rogues and warriors swapping, priests and warlocks always nearly even)
The classes for the Alliance are in this order of popularity: Paladin > Rogue > Hunter > Warrior > Mage > Druid > Priest > Warlock (the first three swapping at times)