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One of the core responsibilities of elmpd-connection--filter is detecting when a complete response has been received. A complete response is concluded with a line of the form "OK" or "ACK [error@command_listNum] {current_command} message_text", so one could check for success by simply examining the last three characters of the current response ("OK\n"). For reasons unknown, the author obtusely chose to use an overly-complex regex instead.
This became an issue when issuing a command that returned approximately 48K of data; each invocation of the regular expression over that response (given a read buffer that was apparently around 4K) took longer & longer until Emacs became completely unresponsive.

Currently, I use string-match, which will become increasingly slow on large responses (imagine a command list that includes a listall followed by a command that fails).

elmpd-connection instances may be normal or "idling"; in the latter case they will invoke the "idle" command on one or more MPD sub-systems, and when any of those sub-systems change, a caller-supplied callback will be invoked with the connection instance and a parameter meant to indicate which subsystems changed. That is presently the MPD response with the final "OK\n" stripped: e.g. "changed: player\nchanged: options\n". Not only is it tedious & repetitive for every callback to parse the text, it forces the callback authors to check for errors/incorrect text. Better to remove the gap between the semantics of this interface ("You will get a list of changed subsystems") and the syntax ("Here's some text-- you have to derive the changed subsystems from that") by, say, having the callback receive a list of symbols.

Hi,

I'm the author of mpdel and libmpdel. Very good naming choice with elmpd. It's refreshing to find someone who is as good as naming things as me :-).

Joke aside, if you want to contribute to libmpdel or mpdel, feel free to do so. I'm using these packages everyday and I regularly merge contributions from others.

Is there a reason why you decided to write your own library instead of building on top of an existing one?

Have fun hacking Emacs!