Curiously enough, even though the harsh vocal approach is a more subjective singing style than clean singing--for instance, harsh vocalists are not open to criticism for singing off-key or being unable to harmonize with backup singers or keyboard/guitar melodies--in truth, like any vocal style, it is possible to make objective distinctions between a “good” / “bad” harsh vocalist.
By my estimation, there are approximately three key, objective criteria that a listener can justifiably use to judge the inherent worth of a harsh vocalist:
Phrasing - i.e. the places where the vocalist chooses to insert his/her lines and its relative affect on a song’s points of emphasis and overall sense of balance; also, the rhythmic style(s) in which the words are sung. For example, because death vocals are a primarily rhythmic instrument, these techniques become a critical part of a band’s success, as death metal in general is mostly a rhythmic/percussive form of metal (appropriately then, bands like Atheist, Cynic, and At the Gates are often considered “progressive,” partially because they chose to place most of their musical emphasis on intricate, interweaving, dual-guitar melodies while placing their respective rhythm sections into a more supportive role).
Emotion – Sticking with the previous example, death metal is intrinsically an intense form of music; therefore, a death metal vocalist must match the emotional intensity of his/her band-mates via the utilization of sounds that more suitably express man’s more viscous and primal emotions (basically, harsh vocals are often the best musical tool for transmitting feelings of vitriol, insanity, torment, etc. whereas clean vocals mesh well with feelings of contentment, calamity, love, etc.)
Relation to musical/lyrical themes – This is the primary, style-related reason why most extreme metal vocalists don’t sing cleanly; again, death metal deals not with the frivolous humanist themes that have historically dominated listener-friendly rock music, but rather, explores--through a primarily nihilistic viewpoint--themes that are typically thought of as taboo in contemporary Western society. Specifically, death metal tells tales of societal corruption, unimaginable violence, and unspeakable evil in an attempt to emphasize the ungoverned brutality of (human) nature. Stated plainly: extreme music demands an equally extreme vocal approach.
Virtually every other aspect of proper, harsh vocal evaluation is related to the personal preferences of the band and its listeners. For instance:
Decipherability/enunciation – Contrary to common opinion, this stylistic decision is a purely subjective aspect of singing, as its ultimate “success” or “failure” is almost always dependent upon the intent of the vocalist/lyricist. These intents vary significantly from band to band thus making it difficult to prove whether or not a certain band has made a “good” decision by choosing a certain level of decipherability/enunciation for their lyrics. In fact, direct consultation with the vocalist/lyricist is often the only way to obtain a correct evaluation of the band’s decision (though they are certainly not a politically motivated musical group, most Atheist songs contain a strong ideological message, and accordingly, Kelly Shaefer’s vocals are much more coherent than the screams/growls of the typical throat-artist from other early-1990s death metal bands).
Variety of style – A lack of vocal variety may or may not be a valid musical mistake depending on several factors such as the vocals’ position in a band’s hierarchy of sonic importance, the overall length of an album, or the vocals’ required adherence to strict thematic guidelines. This variety of vocal purpose often causes ignorant listeners who cannot abandon the “pop-music” mentality--i.e., that a song should be lead by its vocals and that the vocals should always be “pleasing to the ear” and “easy to understand”--to make incorrect assertions regarding vocal quality and overall song/album quality.
Of course, all this text hasn’t even taken into account variations in production styles that can alter the depth and texture of a singer’s voice or emphasize/deemphasize the importance of the vocal performance by lowering it in the mix (this is probaly why the vocals on complex albums like “Unquestionable Presence” are often so low in the mix; the music was created to become the album’s centerpiece). Consequently, if a listener is trying to focus first on the lyrical content in albums like "Unquestionable Presence" and not on the music itself, he/she is ignoring said album's central core of content. Again, most death metal is a guitar and drum driven style of music whose lyrics serve as an appropriate and necessary but relatively less significant aspect of a song’s overall purpose. Simply stated, death metal in its most basic forms is all about riffs and rhythms; any other aspect of the music typically gets relegated to a secondary role.