Let me take a shot at this, and we'll see how others agree/disagree, and what conclusions we can arrive at.
Viking metal is defined by the sound created during Bathory's "Viking" metal era (i.e. Blood Fire Death, Hammerheart, Twilight of the Gods). It typically has a slower sound than black and death metal, although it can definitely be said to derive from black metal, and many Viking metal bands possess black metal qualities. I would posit that keyboards/symphonics play a generally larger role in Viking metal than in most traditional black/death metal. The presence of the keys creates what I believe most of us would designate an "epic" sound. I would also suggest that a mix of clean and harsh vocals are utilized frequently in Viking metal (but aren't necessary). In general, most traditional "Viking" metal bands exhibit a much slower sound than other forms of extreme metal, and strive for a sound that I would describe as "epic" or "glorious" (as cheesy as that sounds).
Bands that I would say qualify as strict Viking metal bands:
Bathory
Einherjer
Spectral
Ancient Rites (although they frequently exhibit strong black metal tendencies)
Fjoergyn
Kromlek
Myrkvar
Black Messiah (although they also lend toward black metal as well)
Thyrfing
Mithotyn
I know that there are more. Anything others that people would like to add to this list? Anything someone disagrees with, from either the list or the description?
EDIT: bands like Ensiferum and Turisas are not Viking metal, but rather fall into the folk metal category, which is why they can be included in this thread. As far as Viking metal goes, I believe that it is a very strict, specific category.