It would be a great thing...for DC.
Marvel never made the sort of amazing creative changes to the status quo we saw from DC in the late 80s and 90s. There'd simply be less stuff for Didio to ruin at Marvel. More importantly, I think that it would be highly unlikely that much of the nostalgia porn DC has produced in the last decade happens without him.
Brubaker jumped ship from DC to Marvel when he was left out of the planning of Infinite Crisis and given high profile work on Captain America. If Didio couldn't understand the talent that he had, why would he try to poach that talent if he didn't? No, in all likelihood, Brubaker stays at DC and we get some more Gotham Central. Furthermore, since Didio's DC hasn't shown much interest in acclaimed talent like Jason Aaron, Matt Fraction and Jonathan Hickman, I don't see Didio's Marvel showing that either. Odds are that they eventually land at DC and maybe even do some Vertigo stuff, leading to a renaissance at that imprint.
So if Didio was running Marvel instead of DC, I see DC crafting a new high watermark for creativity in the medium, while Marvel hands Captain America to JT Krul and Iron Man to Scott Lobdell.
GI Joe Classic Vol 3 by Larry Hama, Mike Vosburg and company
The Mourner by Richard Stark