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)

None of the LCD font smoothing options are going to do you any good, and here's what I suspect is the reason.

Subpixel rendering simply doesn't work with resolution-independence as implemented by Apple. If OS X renders everything on a much larger pixel grid and uses subpixel rendering, it would look fine if shown at native resolution. Instead, in 10.8, the graphics card scales the larger pixel grid down to what's displayable by the monitor. During the downscaling multiple adjacent pixels get averaged together and combined. For black text on a white background, this means that the red, red+green, blue+green, or blue subpixels on the edges of fonts (which give subpixel rendering its characteristic crispness) are averaged with adjacent white pixels during the downscaling. The subpixel effect basically disappears in the process and you're left with entire pixels colored funny around the text, and none of the LCD smoothing options look acceptable. Your best bet is to UNCHECK "use LCD font smoothing" in the System Preferences and let the graphics card antialiasing do the job instead. It's not as good as what we had before, but at least it isn't quite as fuzzy. The only way I could see this being addressed would be if the fonts were rendered on the native pixel grid after scaling instead.

Notice that if you zoom in using the OS X zoom feature, with LCD font rendering enabled, you see colored blocks around the fonts. But if you zoom all the way out and take a photograph of the screen using a camera, subpixel rendering is actually not taking place! I'm a bit surprised that Apple would leave the option in there when it clearly doesn't function as intended with external monitors.

Here is a picture I took of my external display connected to my rMBP with "flat panel" (sub pixel) font smoothing enabled. Notice that it's definitely not occurring, because entire pixels are being dimmed to the sides of characters.