I'm sensing a little hyperbole here, Crimson Longinus, so let's discuss how something can be different but not so wildly different that it feels like a piece of another game. Set the wayback machine to 2004, and Complete Arcane drops. In it is a new base class, the Warlock. The Warlock is a magic user, but he doesn't prepare spells. He doesn't have spell slots. He uses spell-like abilities which resemble spells, can have unique effects. For example, when he dispels magic using his Voracious Dispelling Invocation, he can also deal damage to targets of his Dispel Magic.
His Spell-Like Abilities have no usage limit per diem, his buffs last for 24 hours. He later gains an impressive ability to emulate being a true spellcaster so that he can use all manner of magical items, such as scrolls or wands, and can later even create his own, despite not having the traditional spell requirements to do so.
Despite some early hullaballoo about his Eldritch Blast providing day-long blasting power, he was no more effective than a 5e Rogue (and perhaps a little less so) in combat. The class had a low ceiling of optimization (by itself), the floor was decently high, as, no matter what you did, you could always Eldritch Blast, and while it was never an all star, it was fun to play, unique, and most importantly, didn't destroy the game's balance. In fact, a modified (and in many ways improved) version, the Dragonfire Adept was later printed, and it didn't make that much of a splash either.