what does "too dark" mean

It probably means that the person speaking likes his sound on the warm side of neutral or, that he likes the top end rolled off, but that what he is hearing is too warm or too rolled off FOR HIM. Remember dark to some means warm and to others it means rolled off. You won't find it in Webster.

I would recommend J. Gordon Holt's Sounds Like audio vocabulary compendium.

Dark: "A warm, mellow, excessively rich quality in reproduced sound. The audible effect of a frequency response which is clockwise-tilted across the entire range, so that output diminishes with increasing frequency."

If you are interested in learning more, I have several links in my website's audio section:
Val e-diction

I take it to mean that it is thicker in the bass with a kind of closed in feeling. Vocals and soundstage presentation are probably more recessed. Kinda like a brooding emotional feeling. Probably will not hear "air and space around instruments" as much, and decay and detail may be limited. Just my take on it, but everyone probably defines it differently.

R.

I would call its opposite "too light" or "too white." Not "too bright" as tbg says (in my book, too bright's opposite is too soft or too dull). Very subjective, and JGH's glossary is useless IMO. I think you can have a dark sound without being rolled in the highs as he suggests.

I like the above recommendations and reponses.
To keep it simple however, "too dark" means "DOESN'T SOUND GOOD!"...just like "too burnt", "too sick", or "too ugly" have negative connotations attached.