Some general suggestions for API requests and Server Responses:
-Include current session time left in header.
-Include current limits status in footer (items searched, searches left per day, items downloaded so far, items to download left).
-Consider alternate API method. URL-based, using OAuth or using an encrypted key consisting of client key + authentication token.
-Consider additional alternate communication formats, like JSON (not replacing XML, as that wouldn't make sense, but JSON is pretty common these days and that could attract more devs)
-Return 404s when limits are reached. Not a full page but a *TRUE* 404. Otherwise the client downloads the gzipped html and thinks it's a subtitle.
-No subtitle format has extension .txt. Absolutely noone. You can analyze the content of the TXT and change the extension to the actual extension and format. Use of ".txt" is an aberration perpetrated by lousy and mediocre polish clients.
-Consider converting to Unicode. All relevant players support unicode and would help with problems in encodings.
Some suggestions I'd make for the XMLRPC methods.
-Add a "Version" field and start versioning the methods. Otherwise it's impossible to improve on them without carrying over the cruft from the past. The field would be optional, if it's not included then version 1 of the method is being used. That way existing clients will keep working and won't break.
-Change TryUploadSubtitles so CD# is optional. Batch uploading can contain completely unrelated subtitles and CD# makes no sense.
-Change the TryUploadSubtitles so it includes an identifier for each response (currently it's not serialized, it should use the subhash or a "serial" integer from the original client request. Otherwise it's not easy to match results to original request (especially when the original request is built on the fly)
-TryUploadSubtitles results ideally would be a mix of CheckSubHash and CheckMovieHash when the sub is not present. That's all the info required. I wrote a longer post on this here: viewtopic.php?p=5935
-Allow subtitles to be uploaded with just iMDB, without any matches. I may download correct subtitles for a movie but not matching my file. I could still upload this.
-NFO creation: This sounds too out there, but it would be nice to be able to download a NFO created for a moviehash. I would like this functionality and it could include a link for OpenSubtitles in it, after including movie info an dimdb ID.
Miscellaneous: Statistics should be updated to reflect the actual state of the server. Currently they keep only half. Things I believe are missing in the stats:
Downloaded subtitles by API and Web by user
Uploaded subtitles from API by user
Matched subtitles by user
API usage over time.
API usage by country.