what does "too dark" mean

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."

Clearly this difference in opinion demonstrates the unfortunate truth that these terms are meaningless because they convey no information. The opposite of too bright is too dull, damped, excessive treble or no liveliness. Dark has always meant something else to me its meaning is almost impossible to properly define its like trying to tell someone what an apple tastes like.
I'll try its as if the timbre and texture of the sound is off in a deficient way so that the sound is closed in and collapsed (note not compressed) on itself. I agree that there is an element of boxiness.

Clearly.