I want to buy a Macbook Pro retina and also an external display for when I'm at home. I could of course buy Apple's thunderbolt display, but I'm looking for alternatives. Will every screen be good or do I need to look out for specific specifications because of the retina display? I guess when you are used to a high-res retina screen, the resolution of the external display should also be very high to match the Macbook retina screen?
The external monitor is completely independent of the retina screen. Any monitor would work (via HDMI or Thunderbolt to DVI/VGA adapters).
OS X more or less seamlessly transitions between the two monitors (retina vs. non-retina).
You will not be able to find an external display that matches the "retina" quality. Not even Apple makes a non-laptop display that is retina. Even if you bought a 4k display, it would still be 1x pixels as opposed to retina's 2x.
It won't be as nice as your retina display, but buy a high quality external monitor and you'll be happy with the quality. You generally sit a bit further back from an external display (verses a laptop) so it isn't as bad using a retina/non-retina combo as you'd imagine.
You can set the resolution of the external display independent from that of the retina display.
I personally use a Full-HD display with a VGA->Thunderbold adapter at work and at home a Full-HD display with a DVI->Thunderbold adapter.
I prefer the DVI variant, but this is also because of the generally better display at home.