They're both famous, but not for the right reasons. Aquaman especially has an uphill battle after years of falling victim to every lazy "lol only talks to fish" joke in the book. It's not going to help at box office when everyone thinks your character is a joke. Something like Animal Man, however, does not have that problem.
Both come with their own risks, of course. But I do think it shows some good thinking on DC's part that they're considering both the big name characters and lesser known characters as priorities for the future.
I don't see that as a such a bad thing. Because of that, you'll automatically have people who go see Aquaman just to see how much fail he is. Or, they'll ask their friend who did see it how much fail it was. Either way, the infamy of Aquman's suckage is such that it will capture people's attention.
After that, the trick is to prove them wrong and make the movie awesome. Or they may not even have to go that far and just market it well. Part of Man of Steel's success is the excellent marketing. The very trailers promised that, unlike the snoozefest that was Superman Returns, this one will be full of heart pumping action.
But I do agree that it's good that DC is FINALLY moving past Batman and Superman. I just wish I had assurance that they will be good movies.
They should take the Sherlock approach to adapting Sandman (this is a Sandman thread now) and create 90 minute long mini-movies for every arc.
The problem with sandman is that it's structure is very up and down. You can have an 10+ issue arc centering around one event. Then you have this single out of nowhere issue dedicated to explaining Nada's backstory or introducing Hob. Sometimes it happens in the middle of an ongoing story arc. These are important things, but they often aren't made relevant until much later. It worked for a comic book because you got the story in short 20~ page bursts a month anyway, so a small interruption wasn't a big deal, but it won't work for a movie where every minute is precious and you don't see the next part for 2 years. A single ongoing story arc like a normal TV show seems to be the best option, since that's the closest structure to comic books anyway (you get a short periodic burst of story like in comics)