Update 11.Feb 2015: Found another product, AirServer which allows using a miracast capable Windows 8.1 machine as a Miracast receiver. Not tested yet.
Second screen with touch support
By 2.December 2013, I found a single paid solution that offers this: AirDisplay from Avatron. They have host/master software for OS X and Windows 7/8, and client software for iOS, OSX and most Windows versions. The host uses the displays while the clients yield their displays to the host. Touch input works correct in my tests with Windows 8.1. It requires extremely high data rates - I need over 5MByte/s, which is not only 200x more than RDP or Citrix use, but it's also the limit of most 2.4 GHz WiFi routers. Metro applications don't display on an AirDisplay screen at all. The application costs 20$ per decice and on my devices the software is unstable. But they do satisfy the requirements of the question, unlike all other solutions I came across.
Because I first looked into Miracast, I summarize my Miracast findings so far:
It is possible - and quite simple - to extend the screen of a Windows 8.1 device onto any monitor device wireless by using Miracast. Select the Charms, goto Devices, select Project, then select your Miracast receiver. Source: http://blogs.windows.com/windows/b/windowsexperience/archive/2013/11/12/windows-8-1-on-your-big-screen-with-miracast.aspx
The following video shows how to set up Miracast and it also shows 2 different devices with very different latency: http://www.youtube.com/watch?v=YbUGIjeqnEk
While it is simple to do this from Windows 8.1, many displays with integrated Miracast support have some trick to it - for example the 2013 Sony TVs require the user to be in the menu "Settings->Network->WiFi Direct Settings" when establishing the first Miracast connection, or it won't work.
There are multiple Miracast receivers on the market which can be plugged into the HDMI port of a display device. It seems some Miracast receivers work, others don't, and others again only work with specific firmware versions.
To date, I could not find a software solution that turns a Windows device into a Miracast receiver. On the other hand, I also did not find any reason why this should not be possible. I did find this xda thread where people are looking for a Miracast receiver software. Noone knows of any despite having 3 pages of replies, with the latest reply from 16th of November 2013: http://forum.xda-developers.com/showthread.php?t=2002404&page=3
It's possible with a small amount of tinkering to set up Media Player on one device in receiver mode and then use PlayTo on another device to stream video onto the display of a second PC.