He was THE major creative force behind DC in the late 90's to early 2000's, then Dan Didio stepped in and began openly shitting on his Flash, Legion and Superman work from that era.Waid made it a point to create new stories and villains, whereas Didio's editorial mandate was playing entirely to silver age nostalgia. So Mark had to watch everything he had added to the DC legacy be dismantled and when he asked why he was told to his face it was awful.
Eventually he got fed up and left to head Boom! studios, but that didn't pan out and he went back to Marvel as a writer. Unfortunately he's an ultra-PC hack these days and nothing he writes comes close to his earlier work for DC or Marvel.