Avengers definitely did mess up DC movies because now the Justice League push has made the studio go into overdrive. While I really, really, really want JL, making it the whole purpose for movies going forward has the glaring danger of making said solo movies turn into stepping stones. Kind of like Captain America which was my least favorite of the Phase 1 Marvel movies. It felt like a stepping stone as did Thor to a slightly lesser degree. And I think the weakest part about Iron Man 2 was the fact that they tried to muscle in Fury and Widow's role into the plot just to make the story relevant to the building Avengers story. If DC ends up doing the same things and introducing WW and Flash ONLY to get to the end which is Justice League, they could meet the same criticisms that the final three movies of Phase 1.
Man of Steel will probably get a pass due to going into production before the Avengers set the box office on fire and seems to be made in order to make a GREAT Superman movie. But if they put MoS 2 on the fast track to get it out by 2015 and then smush whoever else they want to get introduced in order to get to their perceived money maker, they could end up losing more of their audience than they gain. DC movie reputation is sort of straddling thin ice right now. The Nolan Batman movies did well but pretty much everything else DC has put out in the last decade has been frowned upon by the GA. MoS will do some good if the reviews stay positive, but they don't want to ruin that directly after.
And that is how the Avengers has thrown WB for a loop.