Comic-Reader Lad and Ascended has already given a great overviews. I don't have much to add to what they've covered.
I'm going to take another tack, with a start from Gail Simone's tweets about him:
I liked Dan very much. We disagreed on a lot. But he did a lot that others took credit for, and when thinks went bad, he often took the blame he didn't deserve.This wasn't something that I knew about him, and it seems he did some good efforts in this area.Dan was very supportive of lgbtq content. If we had something like that that we wanted approved, we made sure to go to Dan over every other editor.
But I also looked at the various comments from other creators, listed by Bleeding Cool, and another trend became apparent.
Gail Simone, Tom King, Evan Dorkin (critical), Bryan Hitch, Liam Sharp, Jennifer de Guzman (critical), Jeff Lamire, Chip Zdarsky, Cecil Castellucci, Van Jensen, Joe Quesada, Dan Jurgens, Kurt Busiek, Cully Hamner, Mitch Gerads, Mark Russell, Jimmy Palmiotti, Donny Cates, Rob Liefeld (critical), James Tynion IV, Alex Segura, Gerry Conway, Jerry Ordway, Brad Meltzer, Joshua Williamson, Frank Tieri, Brad Walker, Scott Snyder, Tim Seeley, Sam Humphries, Shane Davis, David Marquez, Aaron Lopresti, Frank Cho, Christos Gage, Denys Cowan, Marc Andreyko, Oeming, Christopher Butcher, Nick Hanover, Philip Tan, Cara McGee, Tom Taney, Mike Perkins, Aaron Campbell, Arthur Adams, Cheryl Lynn Eaton (semi-critical), Brian Bendis
Notice something about the list? It has a preponderence of dudes. The number of women listed can be counted on one hand (Gail Simone, Jennifer de Guzman, Cecil Castellucci, Cara McGee, and Cheryl Lynn Eaton; though Jimmy Palmiotti speaks for Amanda Conner as well). But the female voices also make up half of the critical ones.
Together with the Berganza situation, and the way New 52 shaped up, I think it's fair to say that DC under DiDio was hostile or at least unwelcoming to most female creators.