USB drive or stick
That's the deal-killer for me.
Streaming from a device like a tablet or laptop to the TV is for the birds except in some circumstances when I just want to show someone a YouTube video or something and don't want to bother downloading the file to a portable USB drive or USB stick first. But, in this day and age to constantly stream entire films from a device like a PC, tablet or smartphone to another gadget that is connected to a TV is a waste of electricity and wears out your (probably expensive) "streaming" host device for no good reason.
I use media players that have a file browser, a remote control and (vital) have a USB port and play a wide range of video formats. If I want to use my tablet as a remote control and for text input I can if it's handy, but the little remote does just fine and the batteries last months.
With this setup I copy HD movies to a USB drive or stick from wherever I want and most of the wear and tear, resources and electricity isn't being drained from my tablet, laptop, etc. just to watch a movie.
What this little Roku and Chromecast needs is a usb port on the end of them or somewhere to allow me to plug a portable USB drive and/or USB stick in them. I still want the wifi so I can remote control it with a basic file browser and also have it be able to stream YouTube videos directly from the Internet, Netflix as well, etc. and still stream from video files on my PC, tablet, etc. on the occasions I want to do that.
Wait a sec, I already have a custom media player that does that... Nevermind.
Of course, the ultimate setup is it does all of the above stuff I mentioned but can also search, acquire and download torrents directly to the media player - but you have to make a custom setup for something like that. AFAIK, you can't buy anything like that off the shelf (someone correct me if I'm wrong).
Edit: I can think of one nice use for a little streaming guy like that Roku or Chromecast. Use them for elaborate pranks on public TVs and broadcast crazy shit. If you have to leave the device you aren't out that much money. Then again, if these devices don't mesh, I guess that's off the table as well unless there's a fast, open wifi or you know the password to a fast wifi hotspot on the premises.