According to the forums (not me), a "Deathmine" is a boxy map in a flatland world that was designed for the sole purpose of being carbon copy of other Deathmines literally because it gets players and players give rates. people who started the game a week ago and get goldenknight through the worst artistic, scripting and all-round design possible.
Most people enjoy putting "RAte 5" signs all around the spawn encased in Rainbow EVERYTHING. Some like competing to see who is the best scripter by putting a really basic script in the map that other maps may not, these are mostly useless scripts and do nothing for the map. the point is, they basically all the look the same in the sense that they all have 4 or 6 basic mines leading up to a 5x3 PvP area over whats called a PvP mine that takes 1 second to get out of. oh btw all the mines are in the same room.
With Deathmines getting so popular, people who host maps under the "Deathmatch" attribute is affected from criticism from others, evening if the map looks and falls under a completely different category. Meaning if they host under the Deathmatch attribute, their map is instantly labelled as "Deathmine" no matter how detailed and RPG looking the map may look. Personally and a few others would contribute to this, but i would love to see that nickname be used in a less prejudice way. Like if i went on someones map and started calling their map terrible because its under the challenge attribute, its just not right.