Font smoothing on external display working badly. (rMBP)
It doesn't look quite as good as my 27 inch iMac, but something that helped me was to turn down the display's built-in sharpness setting to zero (Dell U2713H). This, combined with increasing the brightness, made my display look much better when plugged into my rMBP (running 10.9.1 at time of writing). It's a huge improvement!

(additional note, I'm plugged into the display with mini displayport)