I voted video, but thinking about it some more, I should probably have voted for both audio and video.
The main thing that got me interested in Media Center at all was when I was asking Hendrik for ReplayGain support in LAV Audio, and he said that it was really a feature for the player rather than the decoder, and directed me towards Media Center.
It was the video library management features and tight integration of LAV Filters/madVR/VideoClock which got me to buy Media Center though. Prior to that I was just ripping discs to ISO images in folders, and letting Windows' Libraries feature handle things.
There are other convenience features on the video side which I find very useful, such as automatically downloading videos vis RSS (including sites which require a login) analyzing them, and placing them correctly in my library on its own is a huge thing for me as far as convenience is concerned. (if only it supported YouTube RSS feeds...)
I do use it more for video than audio, but it's probably 60/40 now.
Media Center has got me to take music library management seriously, which has resulted in me spending far more time listening to music and building up a music collection since I bought it.
My library has grown in size considerably due to the amount of music I'm listening to now, and that's likely to continue as I start to take advantage of the zones feature to send audio all around the house.
It irks me that images don't get more love and attention.
Jim even neglected to put them in the poll subject.
MC does pictures too you know, and does them really well.
Until it supports color management (which would also be useful for album art) I
can't use Media Center for photo viewing.
I haven't used a point & shoot camera in almost a decade at this point - none of my photos are sRGB.
Even with color management support, I'm not likely to move away from Lightroom for photographs that I've taken myself though - I'm clearly not the audience JRiver are targeting with their image support. (though I'm pretty sure consumer apps like iPhoto
do have color management support)
What I'm actually most likely to use the images feature for is slideshows when music is playing - there are a few Tumblr sites I pull the images from which would be good candidates for this, but Media Center needs animated .gif support for that to be worthwhile.