To call Goth Metal (allmusic, wikipedia) dark is not the greatest understatement possible, but it's close.  Similar to Doom Metal, Goth strives to create a somber mood, and tends toward much slower tempos than, for example, Speed/Thrash, while also often alternating between loud and soft sections within the composition.  Many Goth vocalists also use Death-style guttural growls in their delivery, although there are a fair number of female Goth vocalists.  There's also a tradition among Goth vocalists to employ the so-called "beauty and the beast" vocal duet technique, combining male Death-style vocals in counterpoint to ethereal female vocals (Black as the Devil Painteth is a good example of this).  Goth lyricism is also some of the more literary among metal subgenres, employing much macabre and religious imagery.