Coloured sockets will take any gem, but if the colours match, the socket will stack another bonus on top of whatever the gem itself does. Meta-Gems do nothing on their own, but add new powers to the other gems in the item.
One might add +3 spell damage for every blue gem fitted, another +4 agility for each differently-coloured gem. Combining these in many-socketed Rare or Epic items is going to add much-needed diversity to high-level gear.Rather than wearing the same set of ultimate Warrior kit as every level 70 Warrior, you'll tailor it to your play style, through your own ingenious gem formulas.
Above: The Blood Elves are mighty handy with magic attacks