Repair-It

Supported since Windows 7, but Windows 8 (and 8.1 additionally) bring refined support for DLNA. You may need uPNP turned on. YMMV. But on the desired target (probably your HTPC), open Media Player (yes, Ed, Media Player) and under the Stream menu, select "Allow remote control of my Player."
Then on any Win 8 device, when WMP is open on the target (important), you can go to your PC Settings and Devices and find the WMP target there. Nothing will work until it appears. Looks like this:
I should note I had some difficulty getting this step to work on my tablet. But eventually it did. I screwed around a lot in Network in This PC...can't say what finally made it work, but it does. The real goal was to be able to use the tablet to queue up things like Youtube videos and play them on the HTPC without having to deal with a mouse/keyboard/web browser on the HTPC itself. Modern technology and all.

Then you can use it as a DLNA play to target from many places. First, let me point out the many things that can send content:

Windows 7 (older support)
Windows 8
Windows 8.1
Windows RT
Windows 8.1 RT
Nokia Lumia phones using Play To app
Probably other devices

Then, from there, you have options. From the Play to Lumia app, you can choose pictures, videos or music.

But from Windows 8.1 (and RT) you have a lot of options. Many apps support play to. IE11 supports play to. Here's the killer use case: browse to youtube, play video, in charms select Devices -> Play and you should be able to select your HTPC running WMP and it will stream to it via DLNA. From my tests quality was great.

So for the HTPC, I recommend launching WMP and setting it to full screen in the background of WMC. When you want to stream something, just close WMC (you can do so from the remote under settings so no BS mouse/keyboard needed for the HTPC, which is the whole point of this exercise). You will get a pause button and transport/scrub controls. It's awesome.

Try it.

Why wouldn't I just play content in WMC on the HTPC?

For me, I wanted to put the What Does a Fox Say? song on youtube on the TV for Ian, using my tablet. It's like smartscreen for Xbox except I don't need to use the Xbox, since my HTPC is already on.

I can see how that might be useful. Except for the uPnP, that is just terrible.

Can't make it work. Going to bed.

Also, turned the firewall off on the HTPC for testing, no change. My Win7 laptop can choose the HTPC as a play-to target from both WMP and the right click context menu. The Play-To window opens but never actually plays the content. Need to try with a Win8 device.

Something is happening, though because WMC stops playing music when I do this. I can also stop WMC/WMP content by clicking stop on the Play-To window, I just can't start anything.

Still doesn't work. Tried from my tablet, I can start video playback from IE11, but it is unreliable. Sometimes it doesn't work (says "connecting" forever), sometimes it just stops at random points in the video. Sometimes the audio/video are out of sync. When I leave WMP open full screen in the background and open and close WMC, WMP is sometimes not full screen. The same holds true for maximized. Also, sometimes when I start playing something WMP comes out of full screen/maximized and plays in a smaller window size.

The main difference between our systems is our HTPC runs Windows 8.1 rather than 7.

I didn't play with it as long but I had no issues on my Dell running 8.1. I will try again.

It won't stay fullscreen. If you maximize it, it should stay maximized. At least it does on Win 7. I'll try that on my Dell, too.

I just watched a couple youtube videos via the Hyper app streamed to my Dell (Win 8.1). Worked flawlessly.

I may catch flak for this but....is your wireless or network somehow at fault here? It clearly works on regular old 2.4GHz 802.11g at my house.

I turned uPNP off and it worked fine. uPNP doesn't do anything on the LAN side of the network anyway. Could be wireless signal strength.

The HTPC is attached via a 1G cable. It might be a signal issue from the tablet. I will check that out later. The playing from Win7 thing not working is a little annoying, though. I will see if there are some DLNA apps for our phones to test out (as that is probably the preferred use case anyway). In a pinch I can always use the (fancy) remote control to use IE11 directly on the HTPC, so I'm not exactly loosing out on the functionality.

Maybe I should get a ChromeCast?

I can hear the exasperated sighs from here....