Awesome! I've missed this from xbmc...
Might I suggest a few improvements from my personal wishlist:
1. Some APIs support checking for subtitles by the files hash. This leads to better subtitles downloaded. Check the API for sublight, opensubtitles or such for more details. It's not hard to get the hash from c#, let me know if I can assist in any way.
2. Would be great if you could support multi-language. I prefer to watch stuff without subs, but some shows have dialogue that is hard to hear, so then subs are great. However, I have friends visiting that don't like english subs, so it would be nice if subs were downloaded for each language defined and appended to the subtitlefile, so that an English subtitle gets named <tv-show title>.eng.srt and a Norwegian <tv-show title>.nor.srt instead of downloading all as <title>.srt. Use ISO 639 country code abbreviations,
http://en.wikipedia.org/wiki/List_of_ISO_639-1_codes for naming the files.
3. Re-download is sweet to be able to force, for specific language(s) or all defined
4. If no hash match could be made, but subs are available for download, it would be nice with a popup to let the user select what to download or cancel the download
That's about it for now, cool thing!