what does "too dark" mean

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.

Potentially deep and rich sounding, which are usually considered positive attributes.

Dark sounding can usually be interpreted where the lower regions of the frequency spectrum are reproduced adequately, but the higher frequencies are not produced with the same weighting. Thereby reproducing what appears to be an unbalanced presentation.

Or more commonly as Drubin put it, rolled off highs.

-IMO

"too" anything is no good. That being said, this audiophool can handle "too dark" much better than "too bright."

Warrenh, a true pearl of wisdom! Nothing irritates me more than the shrill shrewish screeching of a "too bright" system. Well, maybe I'm overstating, since most Agoners have systems which wouldn't send me running out of the room, even if leaning toward the bright end. I, like Warrenh, would be more at ease with a top end that is relaxed vs. one that is aggressive. But, as Paul Simon said,

"One man's ceiling
Is another man's floor."