Dark Ritual was OP because of the abundance of BBB or 2B threats that it could plomp on the table on turn 1, most famous of them being Necropotence, of course, followed closely by Phyrexian Negator or Bad moon and a swarm of black weenies and with honourable mentions to Stupor or Duress + Ravenous Rats (Stupor and Duress not being threats, but still it sucks to lose two cards on turn 1 before you can do anything about it).

It's interesting to note that when the ability to produce a lot of quick mana with a sorcery was shifted to Red, two of them (Seething Song and Rite of Flame) also got banned (in Modern).