By utilizing mob-tagging you can get from 85-90 in around 1 to 2 hours per level.
The mobs are named Weeping Horrors
Weeping Horror - NPC - World of Warcraft
They drop what you'd expect, motes etc and grey items.
My friend whom I power leveled with this was level 86 and was getting just under 13,000 exp per tag kill. The mobs respawn instantly and always respawn in the same little area.
Just figure'd this is a useful exploit, I've seen it being abused. Instant respawns are generally patched quickly, so use this one while you can.