The sound of the album is very noisy and rough, perhaps in part because the band thought they were recording a demo when they recorded the album over a period of only three days. According to author Dayal Patterson, Welcome to Hell might be the first black metal album.
British journalist Geoff Barton stated in his 1981 five-star review of Welcome to Hell that the album had "the hi-fi dynamics of a 50-year-old pizza", and that it "brought a new meaning to the word 'cataclysmic'". According to AllMusic's journalist Eduardo Rivadavia, highlights of the album include "Welcome to Hell", "In League with Satan", "One Thousand Days in Sodom" and "Witching Hour"; Rivadavia said of "Witching Hour": "Possibly Venom's single most important track, in it you'll hear a number of stylistic devices which would later pervade all extreme metal genres, indeed become their most regularly abused clichés." Canadian journalist Martin Popoff wrote that "Welcome to Hell got a certain fabulously stupid impetus to it, despite the sub-bootleg quality recording, and Cronos quickly establishing himself as the most annoying voice in rock"; it should be considered "a record of historical metal relevance", but "not the band's most listenable product".