Contemplate is a very simple AddOn designed to enable enemy nameplates automatically in combat and hide them when not.

There is no configuration necessary, just install and you're good to go.

Note about Patch 4.0.1
There's some general glitchiness with nameplates in 4.0.1 due, as far as I can tell, to Blizzard's new 'bloat' plates, meant to emphasize threat. You can fix some of it by running a console command:

/console bloattest 1

Here are some other variables to play with to modify the nameplates:

/console spreadnameplates [0, 1] - Makes nameplates overlap.
/console bloatnameplates [0, 1] - Makes nameplates larger depending on threat percentage.
/console bloatthreat [0, 1] - Makes nameplates resize depending on threat gain/loss. Only active when a mob has multiple units on its threat table.

Hopefully Blizzard will get nameplates a little more stable in the near future.